Understanding Microbes

in Sickness and in Health

National Institute of Allergy and Infectious Diseases

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

National Institutes of Health

NIAID SCIENCE EDUCATIONNIAID

National Institute of Allergy and Infectious Diseases

Understanding Microbes

in Sickness and in Health

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

National Institutes of Health

National Institute of Allergy and Infectious Diseases

NIH Publication No. 09-4914

September 2009

www.niaid.nih.gov

1NIAID I UNDERSTANDING MICROBES

Contents

What Are Microbes?

Microbes Have Bothered Us for Millennia

Microbes Can Make Us Sick

Microbes Can Infect Us

Some People Are Immune to Certain Diseases

Microbes Cause Different Kinds of Infections

You Can Prevent Catching or Passing on Germs

When You Should Go to the Doctor

Infectious Diseases Are Diagnosed in Many Ways

Infectious Diseases Are Treated in Many Ways

New and Old Microbes Emerge on the Scene

Research

Glossary

More Information

Credits

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(cover) Electromicrograph of an amoeba as it entraps a Legionella pneumophila

bacterium, which causes Legionnaires’ disease. (left) Macrophage infected by 

Fransicella tularensis bacteria, which cause tularemia.

NIAID I UNDERSTANDING MICROBES

3

What Are Microbes?

Microbes are tiny organisms—too tiny to see without a 

microscope, yet they are abundant on Earth. They live 

everywhere—in air, soil, rock, and water. Some live happily 

in searing heat, while others thrive in freezing cold. Some 

microbes need oxygen to live, but others do not. These 

microscopic organisms are found in plants and animals as 

well as in the human body.

Some microbes cause disease in humans, plants, and animals. 

Others are essential for a healthy life, and we could not exist 

without them. Indeed, the relationship between microbes 

and humans is delicate and complex. In this booklet, we will 

learn that some microbes keep us healthy while others can 

make us sick.

Most microbes belong to one of four major groups: bacteria, 

viruses, fungi, or protozoa. A common word for microbes that 

cause disease is “germs.” Some people refer to disease-causing 

microbes as “bugs.” “I’ve got the flu bug,” for example, is 

a phrase you may hear during the wintertime to describe an 

influenza virus infection.

Since the 19th century, we have known that microbes cause 

infectious diseases. Near the end of the 20th century, researchers 

began to learn that microbes also contribute to many chronic 

diseases and illnesses. Mounting scientific evidence strongly 

links microbes to some forms of cancer, coronary artery disease, 

diabetes, multiple sclerosis, and chronic lung diseases.

Note: Words in bold are in the glossary near the end of this booklet.

(left) Streptococcus. There are many kinds of Streptococcus bacteria. Some can cause diseases such as 

strep throat, meningitis, and pneumonia. Others keep us healthy and are found in areas of the human 

body such as the intestines, skin, mouth, and nose.
4 NIAID I UNDERSTANDING MICROBES

Bacteria

Microbes belonging to the bacteria group are made up of only 

one cell. Under a microscope, bacteria look like balls, rods, or 

spirals. Bacteria are so small that a line of 1,000 could fit across 

the eraser of a pencil. Life in any form on Earth could not exist 

without these tiny cells.

(above) Thermophiles are microbes that like the high temperatures found in places like the hot springs of 

Yellowstone National Park.

Scientists have discovered fossilized remains of bacteria that 

date back more than 3.5 billion years, placing them among the 

oldest living things on Earth. Bacteria can inhabit a variety of 

environments, including extremely cold and hot areas.

• Psychrophiles, or cold-loving bacteria, can live in 

temperatures as cold as those found in the Arctic and 

Antarctic.

• Thermophiles are heat-loving bacteria that can live in high 

temperatures, such as in the hot springs of Yellowstone 

National Park in Wyoming.

• Extreme thermophiles, or hyperthermophiles, can thrive at 

235°F near volcanic vents on the ocean floor.
5NIAID I UNDERSTANDING MICROBES

Many bacteria, however, prefer the milder temperature of 

the healthy human body.

Like humans, some bacteria (aerobic bacteria) need oxygen 

to survive. Others (anaerobic bacteria) do not. Amazingly, 

some can adapt to new environments by learning to survive 

with or without oxygen.

Like all living cells, each bacterium requires food for 

energy and building materials. There are countless numbers 

of bacteria on Earth—most are harmless, and many are 

even beneficial to humans. In fact, less than 1 percent of 

bacteria cause diseases in humans. For example, harmless 

anaerobic bacteria, such as Lactobacilli acidophilus, live 

in our intestines, where they help digest food, destroy 

disease-causing microbes, fight cancer cells, and give the 

body needed vitamins.

Healthy food products, such as yogurt, sauerkraut, and 

cheese, are made using bacteria.

Some bacteria, however, produce poisons called toxins, 

which can make us sick. For example, botulism, a severe 

form of food poisoning, affects the nerves and is caused 

by toxins from Clostridium botulinum bacteria.

ARE TOXINS ALWAYS HARMFUL?

Under certain circumstances, 

bacterial toxins can be helpful. 

Several vaccines that keep us from 

getting sick are made from bacterial 

toxins. For example, one type of 

pertussis vaccine, which protects 

infants and children from pertussis 

(whooping cough), contains toxins 

from Bordetella pertussis bacteria. 

This vaccine is safe and effective 

and causes fewer reactions than 

other types of pertussis vaccines. 

(above) A healthcare provider gives a 

child a vaccine.
6 NIAID I UNDERSTANDING MICROBES

Viruses

Viruses are among the smallest microbes, much smaller even 

than bacteria. Viruses, however, are not cells. They consist of one 

or more molecules of DNA or RNA, which contain the virus’ 

genes surrounded by a protein coat. Viruses can be rod-shaped, 

sphere-shaped, or multisided. Some viruses look like tadpoles.

Unlike most bacteria, most viruses do 

cause disease because they invade living, 

normal cells, such as those in your body. 

Once inside the cells, they multiply and 

produce other viruses like themselves. Each 

virus is very particular about which cell it 

attacks. Various human viruses specifically 

attack particular cells in your body’s 

organs, systems, or tissues, such as the liver, 

respiratory system, or blood.

Although types of viruses behave differently, 

most survive by taking over the machinery 

that makes a cell work. Briefly, when a piece 

of a virus, called a virion, comes in contact 

with a cell it likes, it may attach to special landing sites on the 

surface of that cell. From there, the virus may inject molecules 

into the cell, or the cell may swallow the virion. Once inside 

the cell, viral molecules such as DNA or RNA direct the cell 

to make new virus offspring. That’s how a virus infects a cell.

Viruses can even “infect” bacteria. These viruses, called 

bacteriophages, may help researchers develop alternatives 

to antibiotic medicines for preventing and treating bacterial 

infections.

Many viral infections do not result in disease. For example, by 

the time most people in the United States become adults, they 

have been infected by cytomegalovirus (CMV). Most 

people infected with CMV, however, do not 

develop CMV-disease symptoms.

(above) A man with shingles rash 

on his neck. Shingles, or herpes 

zoster, is a painful infection caused 

by varicella-zoster virus, a type of 

Herpesvirus. This virus also causes 

chickenpox. (below) Representation 

of an influenza virus.
7NIAID I UNDERSTANDING MICROBES

Other viral infections can result in deadly diseases such as 

AIDS or Ebola hemorrhagic fever.

Fungi

A fungus is actually a primitive plant. 

Fungi can be found in air, in soil, 

on plants, and in water. Thousands, 

perhaps millions, of different types of 

fungi exist on Earth. The most familiar 

ones to us are mushrooms, yeast, mold, 

and mildew. Some live in the human 

body, usually without causing illness.

Fungal diseases are called mycoses. Mycoses can affect your 

skin, nails, body hair, internal organs such as your lungs, 

and body systems such as your nervous system. Aspergillus 

fumigatus fungi, for example, can cause aspergillosis, a 

lung disease.

Some fungi have made our lives easier. Penicillin and other 

antibiotics, which can kill harmful bacteria in our bodies, are 

made from fungi. Other fungi, such as certain yeasts, also can 

be helpful. For example, when a warm liquid, such as water, 

and a food source, such as sugar, are added to certain yeasts, 

the fungus ferments. The process of fermentation is essential 

for making healthy foods like some breads and cheeses.

Protozoa

Protozoa are a group of microscopic one-celled animals. 

Protozoa can be parasites or predators. In humans, protozoa 

usually cause disease.

Some protozoa, like plankton, live in water environments and 

serve as food for marine animals, such as some kinds of whales. 

Protozoa also can be found on land in decaying matter and 

in soil, but they must have a moist environment to survive. 

Termites wouldn’t be able to do such a good job of digesting 

wood without these microorganisms in their guts.

(above) A photomicrograph of the fungus 

Aspergillus fumigatus.
8 NIAID I UNDERSTANDING MICROBES

Malaria is caused by a protozoan parasite. Another protozoan 

parasite, Toxoplasma gondii, causes toxoplasmosis, or toxo, in 

humans. This is an especially troublesome infection in pregnant 

women because of its effects on the fetus, and in people with 

HIV/AIDS or other disorders of the immune system.

(above) A pregnant woman looks on while her husband changes the cat’s litter. Used litter may contain 

Toxoplasma gondii, a disease-causing parasite that can pass from a mother to her unborn child.

MICROBES IN THE HEALTHY HUMAN BODY*

Ear (outer) Aspergillus (fungus)

Skin Candida (fungus) 

Small intestine Clostridium

Intestines E. coli

Vagina Gardnerella vaginalis

Stomach Lactobacillus

Urethra Mycobacterium

Nose Staphylococcus aureus

Mouth Streptococcus salivarius

Large intestine Trichomonas hominis (protozoa)

*A selection of usually harmless microbes, some of which help keep our bodies functioning normally. 

If their numbers become unbalanced, however, these microbes may make us sick. All are bacteria, 

unless otherwise noted. 
NIAID I UNDERSTANDING MICROBES

9

Microbes Have Bothered 

Us for Millennia

Microbes have probably always caused infection and disease in 

humans. Since ancient times, historians have documented some 

of those diseases. Present-day archeologists and microbiologists 

(scientists who study microbes) are discovering evidence of 

infectious disease in prehistoric human skeletons.

In a fascinating find in the late 20th century, researchers 

uncovered evidence that prehistoric humans were troubled by 

microbial parasites and used natural remedies against them. 

Along with the frozen mummy of the “Ice Man,” who was 

found in the mountains of northern Italy and lived between 3300 

and 3100 B.C., scientists found a type of tree fungus containing 

oils that are toxic to intestinal parasites. Later, in the laboratory, 

researchers found the eggs of a microscopic parasitic intestinal 

roundworm, Trichuria trichiura (whipworm), in his intestines.

Smallpox, which is caused by a variola virus, 

was described in ancient Egyptian and Chinese 

writings. According to some researchers, over 

the centuries smallpox was responsible for 

more deaths than all other infectious diseases 

combined. It killed millions of people over 

thousands of years before being eradicated late 

in the 20th century by worldwide vaccination. 

The last case of naturally occurring smallpox 

was recorded in 1977.

The protozoan parasite Plasmodium causes malaria, a tropical 

disease that usually is transmitted to humans during the bite 

of the Anopheles mosquito. In ancient times, this disease was 

mentioned in Egyptian writings, called hieroglyphics, and was 

described in detail by the Greek physician Hippocrates. Malaria 

(above) Plasmodium falciparum, 

the protozoa that cause some 

cases of malaria. 
10NIAID I UNDERSTANDING MICROBES

ravaged invaders from the Roman Empire. Though rare in 

the United States, malaria remains a serious public health 

threat worldwide.

Evidence on a 1300 B.C. Egyptian stone engraving shows 

that poliomyelitis (polio) also has been around since ancient 

times. In the 1990s, public health officials launched a 

massive international vaccination campaign to eradicate 

the polio virus, which causes paralysis and can be deadly.

SOME SCIENTIFIC EVENTS AND ADVANCES

Approximately 

300 B.C.

Aristotle, 

Greek 

philosopher 

and scientist, 

studied 

and wrote 

about living 

organisms.

1675

Antony van 

Leeuwenhoek 

discovered 

bacteria.

1796

Edward 

Jenner laid the 

foundation 

for developing 

vaccines.

1848

Ignác Fülöp 

Semmelweis 

discovered 

that simple 

handwashing 

could prevent 

passage of 

infection from 

one patient to 

another.

1857

Louis Pasteur 

introduced the 

germ theory 

of disease.

1867

Joseph Lister 

showed 

evidence that 

microbes 

caused disease 

and pioneered 

the use of 

antiseptics 

during surgery 

to kill germs.

1876

Robert Koch, 

by studying 

anthrax, 

showed 

the role of 

bacteria in 

disease.

1928

Alexander 

Fleming was 

credited with 

discovering 

penicillin.
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In the 14th century, a bacterium that modern scientists identified 

as Yersinia pestis caused the bubonic plague, or Black Death. 

Bubonic plague entered Europe and Africa through infected 

rodents and fleas that accompanied travelers along trade routes 

from Mongolia. The plague epidemic spread through Europe, 

Africa, and the Middle East, killing about 20 million people 

in Europe alone. Plague is spread to humans through the 

bites of fleas, which pick up the bacteria while sucking blood 

from rodents, especially rats. In the United States, healthcare 

providers report cases of plague even today, most of which are 

found in the Southwest.

Viruses caused two major pandemics in modern times. 

From 1918 to 1919, the influenza virus ravaged worldwide 

populations. Estimates of the number of people killed during 

the so-called “Spanish flu” pandemic range from 20 million 

to 40 million. HIV, which was identified in 1984 as the cause 

of AIDS, killed an estimated 2 million people worldwide in 

2008 alone.

(above) The Black Death plagued medieval Europe.
12NIAID I UNDERSTANDING MICROBES

Microbes Can Make Us Sick

According to healthcare experts, infectious diseases caused by 

microbes are responsible for more deaths worldwide than any 

other single cause. They estimate the annual cost of medical 

care for treating infectious diseases in the United States alone 

is about $120 billion.

The science of microbiology explores how microbes work 

and how to control them. It seeks ways to use that knowledge 

to prevent and treat the diseases microbes cause. The 20th 

century saw an extraordinary increase in our knowledge about 

microbes. Microbiologists and other researchers had many 

successes in learning how microbes cause certain infectious 

diseases and how to combat those microbes.

Unfortunately, microbes are much better at adapting to new 

environments than are people. Having existed on Earth for 

billions of years, microbes are constantly challenging human 

newcomers with ingenious new survival tactics.

• Many microbes are developing new properties to resist 

drug treatments that once effectively destroyed them. 

Drug resistance has become a serious problem worldwide.

• Changes in the environment have put certain human 

populations in contact with newly identified microbes that 

cause diseases we have never seen before, or that previously 

occurred only in isolated populations.

• Newly emerging diseases are a growing global health 

concern. Since 1976, scientists have identified approximately 

30 new pathogens, such as HIV.
13NIAID I UNDERSTANDING MICROBES

SOME COMMON DISEASES AND INFECTIONS 

AND THEIR MICROBIAL CAUSES

Bacteria Fungus Protozoa Virus

Athlete’s foot s

Common cold s

Diarrheal diseases s s s

Flu s

Genital herpes s

Meningitis s s

Pneumonia s s s

Shingles s

Sinusitis s s

Skin diseases s s s s

Strep throat s

Tuberculosis s

Urinary tract 

infection s

Vaginal infection s s s

Bacteria Fungus Protozoa Virus

(above) Microbiologist studying highly infectious viruses.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Microbes Can Infect Us
 

Below are some of the many ways you can get infected by germs. 

Some microbes can travel 

through the air 

You can transmit microbes to another 

person through the air by coughing 

or sneezing. These are common ways 

to get viruses that cause colds or flu, 

or the bacteria that cause tuberculosis 

(TB). Interestingly, international 

airplane travel can expose you to germs 

not common in your own country. 

(above) Mycobacterium tuberculosis is the 

bacterium that causes most cases of tuberculosis 

in humans. NIAID I UNDerstANDINg MIcrobes 

Close contact can pass germs to another person 

Scientists have identified more than 500 types of bacteria that 

live in our mouths. Some keep the oral environment healthy, 

while others cause problems like gum disease. One way you 

can transmit oral bacteria is by kissing. 

Microbes such as HIV, herpes simplex virus type 2, which causes 

genital herpes, and Neisseria gonorrhoeae bacteria, which causes 

gonorrhea, are examples of germs that you can pass to another 

person through sexual intercourse. 

You can pick up and spread germs by touching 

infectious material 

A common way for some microbes to enter the body, especially 

when caring for young children, is through unintentionally 

passing feces from hand to mouth or the mouths of young 

children. Infant diarrhea is often spread in this way. Daycare 

workers, for example, can pass diarrhea-causing rotavirus or 

Giardia lamblia (protozoa) from one baby to the next between 

diaper changes and other childcare practices. 

14 
15NIAID I UNDERSTANDING MICROBES

It also is possible to pick up cold viruses from shaking 

someone’s hand or from touching contaminated surfaces, such 

as a doorknob or computer keyboard.

A healthy person can carry germs and pass them to others

The story of “Typhoid Mary” is a famous example from 

medical history about how a person can pass germs to others, 

yet not be affected by those germs. The germs in this case were 

Salmonella typhi bacteria, which cause typhoid fever and are 

usually spread through food or water.

In the early 20th century, Mary Mallon, an Irish immigrant, 

worked as a cook for several New York City families. More than 

half of the first family she worked for came down with typhoid 

fever. Through a clever deduction, a researcher determined that 

the disease was caused by the family cook. He concluded that 

although Mary had no symptoms of the disease, she probably 

had had a mild typhoid infection sometime in the past. Though 

not sick, she still carried the Salmonella bacteria and was able to 

spread them to others through the food she prepared.

HOW IS TYPHOID FEVER SPREAD?

Salmonella typhi bacteria live only in 

humans—in their bloodstreams and 

intestinal tracts. In addition, a small 

number of people, called carriers, 

get better after having typhoid fever, 

but continue to be infected (carry the 

bacteria). People who are infected or 

who have the disease can shed S. typhi

in their feces.

You can get typhoid fever if you eat 

food or drink liquids that have been 

prepared by someone who has typhoid 

fever or who is infected with S. typhi. 

Untreated sewage also can contain the 

bacteria, which can then get into the 

water you drink or use to wash food.

(above) Preparing food properly helps get 

rid of microbes that may cause a foodborne 

illness.
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Germs from your household pet can make you sick 

You can catch a variety of germs from animals, especially 

household pets. The rabies virus, which can infect cats and 

dogs, is one of the most serious and deadly of these microbes. 

Fortunately, rabies vaccine prevents animals from getting 

rabies. Vaccines also protect people from accidentally getting 

the virus from an animal. In addition, vaccines prevent people 

who already have been exposed to the virus, such as through 

an animal bite, from getting sick.

SELECTED DISEASES WE CAN GET DIRECTLY 

OR INDIRECTLY FROM ANIMALS

Anthrax

BSE (bovine spongiform encephalopathy, also called mad cow disease)

Brucellosis

Cat scratch disease

Cryptosporidiosis

Flu

Giardiasis

Hantavirus pulmonary syndrome

Histoplasmosis

Hookworm

Listeriosis

Rabies

Ringworm

Salmonellosis

SARS (severe acute respiratory syndrome)

Toxoplasmosis

Tularemia
17NIAID I UNDERSTANDING MICROBES

Dog and cat saliva can contain any of more than 100 

different germs that can make you sick. Pasteurella

bacteria, the most common, can be transmitted through 

bites that break the skin, causing serious, and sometimes 

fatal, diseases such as meningitis. Meningitis is the 

inflammation of the lining of the brain and spinal cord.

Warm-blooded animals are not the only ones that can cause you 

harm. Pet reptiles such as turtles, snakes, and iguanas can give 

Salmonella bacteria to their unsuspecting owners.

You can get microbes from tiny critters 

Mosquitoes may be the most common carriers, also called 

vectors, of pathogens. Anopheles mosquitoes can pick up 

Plasmodium, which causes malaria, from the blood of an infected 

person and transmit the protozoan to an uninfected person.

Fleas that pick up Yersinia pestis bacteria from rodents can then 

transmit plague to humans.

Ticks, which are more closely related to crabs than to insects, 

are another common vector. The tiny deer tick can infect 

humans with Borrelia burgdorferi, the bacterium that causes 

Lyme disease, which the tick picks up from mice.

MOSQUITO

• Dengue fever

• West Nile virus 

infection

• Viral encephalitis

FLEA

• Plague

• Typhus

TICK

• Babesiosis

• Lyme disease

• Rocky Mountain 

spotted fever

• Tularemia

(above) Mosquito, flea, and tick and some of the diseases they can transmit. (upper right) A turtle.
18NIAID I UNDERSTANDING MICROBES

Some microbes in food or water could make you sick 

Every year, millions of people worldwide become ill from eating 

contaminated foods. Although many cases of foodborne illness 

or “food poisoning” are not reported, the Centers for Disease 

Control and Prevention (CDC) estimates there are 76 million 

cases of such illnesses in the United States each year. In addition, 

CDC estimates 325,000 hospitalizations and 5,000 deaths are 

related to foodborne diseases each year. Microbes can cause 

these illnesses, some of which can be fatal if not treated properly.

Poor manufacturing processes or poor food preparation can allow 

microbes to grow in food and subsequently infect you. Escherichia 

coli (E. coli) bacteria sometimes persist in food products such as 

undercooked hamburger meat and unpasteurized fruit juice. These 

bacteria can have deadly consequences in vulnerable people, 

especially children and the elderly.

Cryptosporidia are bacteria found 

in human and animal feces. These 

bacteria can get into lake, river, and 

ocean water from sewage spills, animal 

waste, and water runoff. Millions 

can be released from infectious fecal 

matter. People who drink, swim in, or 

play in infected water can get sick.

Anyone, including babies, with diarrhea caused by Cryptosporidia

or other diarrhea-causing microbes, such as Giardia and 

Salmonella, can infect others while using swimming pools, 

waterparks, hot tubs, and spas.

Transplanted animal organs may harbor germs

Researchers are investigating the possibility of transplanting 

animal organs, such as pig hearts, into people. They, however, 

must guard against the risk that those organs also may transmit 

microbes that were harmless to the animal into humans, where 

they may cause disease.

(above) Scientist sampling pollution levels in 

a creek.
Some People Are Immune to 

Certain Diseases 

We become immune to germs through natural and artificial 

means. As long ago as the 5th century B.C., Greek doctors 

noticed that people who had recovered from the plague would 

never get it again—they seemed to have become immune or 

resistant to the germ. You can become immune, or develop 

immunity, to a microbe in several ways. 

The first time T cells and B cells in your immune system meet 

up with an antigen, such as a virus or bacterium, they prepare 

the immune system to destroy the antigen. Because the immune 

system often can remember its enemies, those cells become 

active if they meet that particular antigen again. This is called 

naturally acquired immunity. 

(above) t cell. 

NIAID I UNDerstANDINg MIcrobes 

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20NIAID I UNDERSTANDING MICROBES

(above) A baby can acquire immunity through breastfeeding. (below and right) Representation of an 

influenza virus.

Another example of naturally acquired immunity occurs when 

a pregnant woman passes antibodies to her unborn baby. 

Babies are born with weak immune responses, but they are 

protected from some diseases for their first few months of 

life by antibodies received from their mothers before birth. 

Babies who are nursed also receive antibodies from breast 

milk that help protect their digestive tracts.

Artificial immunity can come from vaccines. Immunization

with vaccines is a safe way to get protection from germs. Some 

vaccines contain microorganisms or parts of microorganisms 

that have been weakened or killed. If you get this type of 

vaccine, those microorganisms (or their parts) will start your 

body’s immune response, which will demolish the foreign 

invader but not make you sick. This is a type of artificially 

acquired immunity.

Immunity can be strong or weak and short- or long-lived, 

depending on the type of antigen, the amount of antigen, and 

the route by which it enters your body. When faced with the 

same antigen, some people’s immune system will respond 

forcefully, others feebly, and some not at all.

The genes you inherit also can influence your likelihood of 

getting a disease. In simple terms, the genes you get from your 

parents can influence how your body reacts to certain microbes.
21NIAID I UNDERSTANDING MICROBES

Microbes Cause Different 

Kinds of Infections

Some disease-causing microbes can make you very sick quickly 

and then not bother you again. Some can last for a long time and 

continue to damage tissues. Others can last forever, but you won’t 

feel sick anymore, or you will feel sick only once in a while. Most 

infections caused by microbes fall into three major groups.

• Acute infections

• Chronic infections

• Latent infections

Acute infections

Acute infections are usually severe and last a short time. 

They can make you feel very uncomfortable, with signs and 

symptoms such as tiredness, achiness, coughing, and sneezing. 

The common cold is such an infection. The signs and symptoms 

of a cold can last for 2 to 24 days (but usually a week), though 

it may seem like a lot longer. Once your body’s immune system 

has successfully fought off one of the many different types 

of rhinoviruses or other viruses that may have caused your 

cold, the cold doesn’t come back. If you get another cold, it’s 

probably because you have been infected with other coldcausing viruses.

Chronic infections

Chronic infections usually develop from acute infections and 

can last for days to months to a lifetime. Sometimes people 

are unaware they are infected but still may be able to transmit 

the germ to others. For example, hepatitis C, which affects the 

liver, is a chronic viral infection. In fact, most people who have 

been infected with the hepatitis C virus don’t know it until they 

have a blood test that shows antibodies to the virus. Recovery 
22NIAID I UNDERSTANDING MICROBES

from this infection is rare—about 85 percent of infected persons 

become chronic carriers of the virus. In addition, serious signs 

of liver damage, like cirrhosis or cancer, may not appear until as 

long as 20 years after the infection began.

Latent infections

Latent infections are “hidden” or “silent” and may or may not

cause symptoms again after the first acute episode. Some infectious

microbes, usually viruses, can “wake up”—become active again 

but not always causing symptoms—off and on for months or 

years. When these microbes are active in your body, you can 

transmit them to other people. Herpes simplex viruses, which 

cause genital herpes and cold sores, can remain latent in nerve 

cells for short or long periods of time, or forever.

Chickenpox is another example of a latent 

infection. Before the chickenpox vaccine became 

available in the 1990s, most children in the 

United States got chickenpox. After the first acute 

episode, usually when children are very young, the 

varicella-zoster virus goes into hiding in the body. 

In many people, it emerges many years later when 

they are older adults and causes a painful disease 

of the nerves called herpes zoster, or shingles.

Researchers are studying what turns these 

microbial antics off and on and are looking 

for ways to finally stop the process.

THE DIFFERENCE BETWEEN DISEASE AND INFECTION

A disease occurs when cells or molecules in your body stop working 

properly, causing symptoms of illness. Many things can cause a disease, 

including altered genes, chemicals, aging, and infection. An infection

occurs when a microbe enters your body and begins to reproduce. The 

invading microbe can directly damage cells, or the immune system can 

cause symptoms such as fever, as it tries to rid your body of the invader. 

Some infections don’t cause disease because the microbe is killed quickly 

or it remains dormant (asleep). 

(above) An electromicrograph 

of a varicella-zoster virus, 

which causes chickenpox and 

shingles.
23NIAID I UNDERSTANDING MICROBES

You Can Prevent Catching or 

Passing on Germs

Handwashing

Handwashing is one of the simplest, easiest, 

and most effective ways to prevent getting 

or passing on many germs. Amazingly, it is 

also one of the most overlooked. Healthcare 

experts recommend scrubbing your hands 

vigorously for at least 15 seconds with 

soap and water, about as long as it takes to 

recite the English alphabet. This will wash 

away cold and flu viruses and staph and 

strep bacteria as well as many other diseasecausing microbes.

It is especially important to wash your hands

• Before preparing or eating food

• After coughing or sneezing

• After using the bathroom

• After changing a diaper

Healthcare providers should be especially conscientious about 

washing their hands before and after examining any patient. 

Workers in childcare and eldercare settings, too, should be 

vigilant about handwashing around those in their care.

Medicines

There are medicines on the market that help prevent you from 

getting infected by germs. For example, you can prevent getting 

the flu by taking an antiviral medicine. Vaccines, however, are 

the best defense against influenza viruses.

(above) Washing your hands well 

is an excellent way to help prevent 

passing disease-causing microbes 

to yourself and others.
24NIAID I UNDERSTANDING MICROBES

Under specific circumstances, your healthcare provider may 

prescribe antibiotics to protect you from getting certain 

bacteria such as Mycobacterium tuberculosis, which causes 

TB. Healthcare experts usually advise people traveling to areas 

where malaria is present to take antiparasitic medicines to 

prevent possible infection.

Vaccines

In 1796, Edward Jenner laid the foundation for modern 

vaccines by discovering one of the basic principles of 

immunization. He had used a relatively harmless microbe, 

cowpox virus, to bring about an immune response that would 

help protect people from getting infected by the related but 

deadly smallpox virus.

Dr. Jenner’s discovery helped researchers find ways to ease 

human disease suffering worldwide. By the beginning of the 

20th century, doctors were immunizing patients with vaccines 

for diphtheria, typhoid fever, and smallpox.

Today, safe and effective vaccines prevent childhood diseases, 

including measles, whooping cough, chickenpox, and the form of 

meningitis caused by Haemophilus influenzae type B (Hib) virus.

Vaccines, however, are not only useful for young children. 

Adolescents and adults should get vaccinated regularly for 

(above) Edward Jenner vaccinates his son.
25NIAID I UNDERSTANDING MICROBES

tetanus and diphtheria. A vaccine to prevent meningococcal 

meningitis is recommended for all adolescents. In addition, 

adults who never had diseases such as measles or chickenpox 

during childhood or who never received vaccines to prevent 

them should consider being immunized. Childhood diseases 

can be far more serious in adults.

If you travel or plan to travel outside the United States, getting 

the immunizations that are recommended for your destination(s) 

is very important. Vaccines can prevent yellow fever, polio, 

typhoid fever, hepatitis A, cholera, rabies, and other diseases 

that are more prevalent abroad than in this country.

SOME VACCINE-PREVENTABLE INFECTIOUS DISEASES

Bacterial meningitis

Chickenpox

Cholera

Diphtheria

Haemophilus influenzae type B (Hib)

Hepatitis A

Hepatitis B

Influenza (flu)

Measles

Mumps

Pertussis (whooping cough)

Pneumococcal pneumonia

Polio

Rabies

Shingles

Tetanus (lockjaw)

Yellow fever 

(above) Vaccine vials.
26NIAID I UNDERSTANDING MICROBES

When You Should Go to the Doctor

Generally, you should consult your healthcare provider if you 

have or think you may have an infectious disease. These trained 

professionals can determine whether you have been infected, 

determine the seriousness of your infection, and give you the 

best advice for treating or preventing disease.

Some infectious diseases, such 

as the common cold, usually 

do not require a visit to a 

healthcare provider. They often 

last a short time and are not 

life-threatening, or there is no 

specific treatment. We’ve heard 

the advice to rest and drink 

plenty of liquids to treat colds. 

Unless there are complications, 

most people with colds find 

that their immune systems 

successfully fight off the viral 

culprits. In fact, the coughing 

and sneezing that make you 

feel miserable are part of 

your immune system’s way of 

fighting off those culprits.

If, however, you have other conditions in which your immune 

system doesn’t function properly, you should be in contact with 

your healthcare provider whenever you suspect you have any 

infectious disease, even the common cold. Such conditions can 

include asthma and immune deficiency diseases like HIV/AIDS.

In addition, some common, usually mild infectious diseases, 

such as chickenpox or seasonal flu, can cause serious harm in 

very young children and the elderly.

YOU SHOULD CALL A 

HEALTHCARE PROVIDER 

IMMEDIATELY IF...

• You have been bitten by an animal

• You are having difficulty breathing

• You have a cough that has lasted for 

more than a week

• You have a fever higher than 100oF

• You have episodes of rapid 

heartbeat

• You have a rash (especially if you 

have a fever at the same time)

• You have swelling

• You suddenly start having difficulty 

with seeing (blurry vision, for 

example)

• You have been vomiting 
27NIAID I UNDERSTANDING MICROBES

Infectious Diseases Are 

Diagnosed in Many Ways

Sometimes your healthcare provider can diagnose an infectious 

disease by listening to your medical history and doing a physical 

exam. For example, listening to you describe what happened 

and any symptoms you have noticed plays an important part in 

helping your doctor find out what’s wrong.

Blood and urine tests are other ways to diagnose an infection. 

A laboratory expert can sometimes see the offending microbe 

in a sample of blood or urine viewed under a microscope. One 

or both of these tests may be the only way to determine what 

caused the infection, or they may be used to confirm a diagnosis.

In another type of test, your healthcare provider will take a 

sample of blood or other body fluid, such as vaginal secretion, 

and then put it into a special container called a Petri dish to see 

whether any microbe “grows.” This test is called a culture. Lab 

workers usually can identify certain bacteria, such as chlamydia, 

and viruses, such as herpes simplex, using this method.

X-rays, scans, and biopsies (taking a tiny sample of tissue from 

the infected area and inspecting it under a microscope) are 

among other tools your healthcare provider can use to make 

an accurate diagnosis.

All of the above procedures are relatively safe, and some can 

be done in your healthcare provider’s office or a clinic. Others 

pose a higher risk to you because they involve procedures that 

go inside your body. One such invasive procedure is taking a 

biopsy from an internal organ. For example, one way a doctor 

can diagnose Pneumocystis carinii pneumonia, a lung disease 

caused by a fungus, is by doing a biopsy on lung tissue and then 

examining the sample under a microscope.

(left) A stethoscope is one of many tools a healthcare provider can use to help diagnose the cause of 

an illness.
28NIAID I UNDERSTANDING MICROBES

Infectious Diseases Are 

Treated in Many Ways

How an infectious disease is treated depends on the microbe that 

caused it and sometimes on the age and medical condition of the 

person who is sick. Certain diseases are not treated at all, but are 

allowed to run their course, with the immune system doing its job 

alone. Some diseases, such as the common cold, are treated only 

to relieve the symptoms. Others, such as strep throat, are treated 

to destroy the offending microbe as well as to relieve symptoms.

By your immune system

Your immune system has an arsenal of ways to fight off 

invading microbes. Most begin with B and T cells and 

antibodies whose sole purpose is to keep your body 

healthy. Some of these cells sacrifice their lives to rid 

you of disease and restore your body to a healthy 

state. Some microbes normally present in your 

body also help destroy microbial invaders. 

For example, normal, good bacteria, such as 

lactobacillus in your digestive system, help 

destroy germs that find their way there.

Other important ways your body reacts 

to an infection include fever and coughing 

and sneezing.

Fever

Fever is one of your body’s special ways 

of fighting an infectious disease. Many 

microbes are very sensitive to temperature 

changes and cannot survive in temperatures 

higher than normal body heat, which is usually around 98.6oF. 

Your body uses fever to destroy influenza viruses, for example.

(above) Red blood cells 

and a white blood cell 

(leukocyte). B and T cells 

are lymphocytes, one kind 

of leukocyte.
29NIAID I UNDERSTANDING MICROBES

Coughing and sneezing

Another tool in your immune 

system’s reaction to invading 

disease-causing microbes is mucus 

production. Coughing and sneezing 

help mucus move those germs out of 

your body efficiently and quickly.

Other methods your body may use to 

fight off an infectious disease include

• Inflammation

• Vomiting

• Diarrhea

• Fatigue

• Cramping

By your healthcare provider

For bacteria

The last century saw an explosion in our knowledge about how 

microbes work and in our methods of treating infectious diseases. 

For example, the discovery of antibiotics to treat and cure many 

bacterial diseases was a major breakthrough in medical history. 

Healthcare providers, however, sometimes prescribe antibiotics 

unnecessarily for a variety of reasons, including pressure from 

patients with viral diseases such as the flu.

Because antibiotics have been prescribed too often or prescribed 

for the wrong diseases for many years, some bacteria have 

become resistant to the killing effects of these drugs. This 

resistance, commonly called antibiotic or drug resistance, has 

become a very serious problem, especially in hospital settings.

(above) A sneeze captured in progress. Droplets 

from sneezing can contain germs that can infect 

another person several feet away.
30NIAID I UNDERSTANDING MICROBES

Bacteria that are not killed by the antibiotic become strong 

enough to resist the same medicine the next time it is given. 

Because bacteria multiply so rapidly, changed or mutated 

bacteria that resist antibiotics will quickly outnumber those 

that can be destroyed by those same drugs.

For viruses

Viral diseases can be very difficult to treat because viruses 

live inside your body’s cells, where they are protected from 

medicines in the bloodstream. Researchers developed the first 

antiviral drug in the late 20th century. The drug, acyclovir, 

was first approved by the Food and Drug Administration 

to treat herpes simplex virus infections. Only a few other 

antiviral medicines are available to prevent and treat viral 

infections and diseases.

Healthcare providers treat HIV infection with a group of 

powerful medicines that can keep the virus in check. Known as 

highly active antiretroviral therapy, or HAART, this treatment 

has improved and lengthened the lives of many suffering from 

this deadly infection.

Viral diseases should never

be treated with antibiotics. 

Sometimes a person with a viral 

disease will develop a bacterial 

disease as a complication of the 

initial viral disease. For example, 

children with chickenpox often 

scratch the itchy skin lesions 

(sores) caused by the virus. 

Bacteria such as Staphyloccocus

can enter those lesions and cause a 

bacterial infection. The healthcare 

provider may then prescribe an 

antibiotic to destroy the bacteria. 

The antibiotic, however, will not 

destroy the chickenpox virus, only 

the staph bacteria.

(above) Drinking water and other fluids can 

help reduce the loss of body fluids due to a 

fever, even a mild one.
31NIAID I UNDERSTANDING MICROBES

Although safe and effective treatments and cures for most 

viral diseases have eluded researchers, there are safe vaccines 

to protect us from viral infections and diseases.

For fungi

Medicines applied directly to the infected area are available 

by prescription and over the counter to treat skin and nail 

fungal infections. Unfortunately, many people have had limited 

success with them. Very powerful oral antifungal medicines 

are available only to treat systemic (within the body) fungal 

infections, such as histoplasmosis. Healthcare providers usually 

prescribe oral antifungal medicines with caution because all of 

them, even the milder ones for skin and nail fungi, can have 

very serious side effects.

(above) Oncychomycosis (fungal infection of the nail) caused by Trichophyton rubrum. T. rubrum also can 

cause athlete’s foot, jock itch, and ringworm.

For protozoa

Diseases caused by protozoan parasites are among the leading 

causes of death and disease in tropical and subtropical regions 

of the world. Developing countries within these areas contain 
32NIAID I UNDERSTANDING MICROBES

three-quarters of the world’s population, and their people 

suffer the most from these diseases. Currently, there are no 

vaccines that can control parasitic diseases.

In many cases, controlling the insects that transmit these 

diseases is difficult because of pesticide resistance, concerns 

regarding environmental damage, and lack of adequate public 

health systems to apply existing insect-control methods. Thus, 

disease control relies heavily on the availability of medicines. 

Healthcare providers usually use antiparasitic medicines to 

treat protozoal infections. Unfortunately, there are very few 

medicines that fight protozoa, and some of those are either 

harmful to humans or are becoming ineffective.

The fight against the protozoan Plasmodium falciparum, the 

cause of the most deadly form of malaria, is a good example. 

This protozoan has become resistant to most of the medicines 

currently available to destroy it. A major focus of malaria 

research is on developing a vaccine to prevent people from 

getting the disease in the first place. In the meantime, many 

worldwide programs hope to eventually control malaria by 

keeping people from contact with infected mosquitoes or 

preventing infection if contact can’t be avoided.

(above) The Anopheles mosquito can transmit Plasmodium, protozoa that cause malaria.
33NIAID I UNDERSTANDING MICROBES

New and Old Microbes

Emerge on the Scene

With the arrival of antibiotics and modern vaccines, as well as 

improved sanitation and hygiene, many diseases that formerly 

posed an urgent threat to public health were brought under 

control or largely eliminated. As a result, by the mid-20th 

century some scientists thought that medical science had 

conquered infectious diseases.

Despite these public health advances, 

new microbes emerge and old microbes 

re-emerge just as they have throughout 

history. Several pressures contribute to 

the emergence of diseases, such as

• Redistribution of human populations

• Rapid global travel

• Changes in the way we use land

• Ecological, environmental, and 

technological changes

Practices such as the misuse of antibiotic medicines also 

contribute to disease emergence. 

In addition, unsanitary conditions in animal agriculture and 

increasing commerce in exotic animals have contributed to the 

rise in opportunity for animal microbes to jump from animals 

to humans. From time to time, with the right combination 

of selective pressures, a formerly harmless microbe found in 

humans or animals can evolve into a pathogen that can cause a 

major outbreak of human disease. These pressures are shaping 

the evolution of microbes and bringing people into closer and 

more frequent contact with pathogens.

(above) Rapid global travel contributes to 

the emergence of new diseases.
34NIAID I UNDERSTANDING MICROBES

Emerging microbes

Scientists usually define emerging microbes as those that have 

appeared only recently in a population or have existed but are 

rapidly increasing in incidence or geographic range. Recent 

examples of such disease-causing microbes are methicillinresistant Staphylococcus aureus (MRSA) bacteria, West Nile 

virus, and 2009 H1N1 influenza virus.

SOME NEWLY EMERGING/EMERGED MICROBES

2009 H1N1 influenza virus

Ebola virus

H5N1 avian influenza virus

Marburg virus

MRSA bacteria

Multidrug-resistant and 

extensively drug-resistant 

TB bacteria

Nipah virus

SARS virus

West Nile virus

(above) A fruit bat (flying fox). Fruit bats in India and 

Malaysia have been found to carry Nipah virus.

Re-emerging microbes

The reappearance of microbes that had been conquered 

successfully or controlled by medicines and vaccines is 

distressing to the scientific and medical communities, as well 

as to the public. One major cause of disease re-emergence is 

that many microbes responsible for causing these diseases 

are becoming resistant to the drugs used to treat them. 

Some examples of re-emerging infectious diseases that are 

of significant public health concern are dengue, malaria, TB, 

and polio.
35NIAID I UNDERSTANDING MICROBES

Research

The National Institute of Allergy and 

Infectious Diseases (NIAID), part of the 

National Institutes of Health (NIH), is 

the federal government’s lead agency for 

conducting and funding research on many 

infectious diseases, including their causes, 

diagnoses, treatments, and prevention 

methods. Biomedical research supported 

by NIAID provides the tools necessary 

to develop diagnostic tests, new and 

improved treatments, vaccines, and other 

means to combat the microbial threats of 

today and tomorrow.

NIAID research activities include:

• Projects to sequence the whole or partial genomes of a 

variety of pathogenic microbes. These projects should help 

scientists understand how the organisms cause disease and 

identify new drugs and vaccines.

• A broad malaria research program. This program is 

conducted by scientists at institutions throughout the United 

States and in several countries where malaria is endemic, 

and by scientists working in NIAID laboratories. NIAID 

and other parts of NIH also participate in the Multilateral 

Initiative on Malaria, a global group that boosts international 

collaboration among malaria scientists and identifies 

resources to enhance malaria research.

• Research on the basic biology of influenza viruses and on 

efforts to find more effective vaccines and treatments for 

influenza.

• Clinical trials involving several experimental HIV vaccines. 

Since 1987, NIAID scientists and grantees have conducted 

(above) Lab work.
36NIAID I UNDERSTANDING MICROBES

trials on more than 67 vaccine candidates and 18 types of 

adjuvants—substances added to vaccines to improve the 

immune system response so that less vaccine is needed to 

provide protection. In 2009, a major NIAID-supported 

HIV vaccine study in Thailand revealed a candidate vaccine 

to be 31 percent effective at preventing HIV infection—

the first time an HIV vaccine has shown any ability to 

prevent infection in people. NIAID continues to build upon 

scientific findings by both conducting basic research to better 

understand the virus and developing and testing promising 

HIV vaccine candidates.

• Three research groups to learn more about emerging 

viruses. By learning how these viruses work, researchers 

hope to develop better ways to diagnose and treat the 

diseases they cause.

• Research on the pathogenesis of bacterial and viral sexually 

transmitted infections and prevention strategies to control 

these infections.

Institute researchers work closely with other agencies, 

institutions, and scientists from across the United States 

and around the world to achieve the common goal of 

controlling and eliminating disease-causing microbes and 

infectious diseases. 

You can get information on current NIAID research activities 

on the Institute Web site at www.niaid.nih.gov.

(above) Lab work.
37NIAID I UNDERSTANDING MICROBES

Glossary

antibiotics—medicines that damage or kill bacteria and are used 

to treat some bacterial diseases.

antibodies—molecules (also called immunoglobulins) produced 

by a B cell in response to an antigen. When an antibody attaches 

to an antigen, it destroys the antigen.

antigen—a substance or molecule that is recognized by the 

immune system. The molecule can come from foreign materials 

such as bacteria or viruses.

B cells—small white blood cells crucial to the immune defenses. 

Also known as B lymphocytes, they come from bone marrow 

and develop into blood cells called plasma cells, which are the 

source of antibodies.

cell—the smallest unit of life; the basic living unit that makes 

up tissues.

disease—a state in which a function or part of the body is no 

longer in a healthy condition.

DNA (deoxyribonucleic acid)—a complex molecule found in the 

cell nucleus that contains an organism’s genetic information.

epidemic—a disease outbreak that affects many people in a 

region at the same time.

genes—units of genetic material (DNA) that carry the directions 

a cell uses to perform a specific function.

genomes—all of an organism’s genetic material. A genome is 

organized into specific functional units called genes.

HIV (human immunodeficiency virus)—the virus that causes AIDS.
38NIAID I UNDERSTANDING MICROBES

immune response—reaction of the immune system to foreign 

invaders such as microbes.

immune system—a complex network of specialized cells, 

tissues, and organs that defends the body against attacks by 

disease-causing microbes.

immunity—protection from germs.

immunization—vaccination or other process that induces 

protection (immunity) against infection or disease caused by 

a microbe.

infection—a state in which disease-causing microbes have 

invaded or multiplied in body tissues.

infectious diseases—diseases caused by microbes that can be 

passed to or among humans by several methods.

inflammation—an immune system process that stops the 

progression of disease-causing microbes, often seen at the site 

of an injury like a cut. Signs include redness, swelling, pain, 

and heat.

microorganisms—microscopic organisms, including bacteria, 

viruses, fungi, plants, and animals.

microscopic—too small to be seen with the naked eye.

molecules—the smallest physical units of a chemical substance 

that still keep the chemical properties of that substance; 

molecules are the building blocks of a cell. Some examples are 

proteins, fats, carbohydrates, and nucleic acids.

organisms—individual living things.

pandemics—diseases that affect many people in different regions 

around the world.
39NIAID I UNDERSTANDING MICROBES

parasites—plants or animals that live, grow, and feed on or 

within another living organism.

pathogens—disease-causing organisms.

protein—a chain of small chemical compounds called amino 

acids.

RNA (ribonucleic acid)—a complex molecule found in the cell 

cytoplasm and nucleus. One function of RNA is to direct the 

building of proteins.

rotavirus—a group of viruses that can cause digestive problems 

and diarrhea in young children.

T cells—small white blood cells (also known as T lymphocytes) 

that direct or directly participate in immune defenses.

tissues—groups of similar cells joined to perform the same 

function.

toxins—agents produced by plants and bacteria, normally very 

damaging to human cells.

vaccines—substances that contain parts of antigens from an 

infectious organism. By stimulating an immune response (but 

not disease), they protect the body against subsequent infection 

by that organism.
40NIAID I UNDERSTANDING MICROBES

More Information

You can get more in-depth information on microbes and 

infectious diseases from a local library or a healthcare provider. 

Other sources of information include the following.

National Institute of Allergy and Infectious Diseases 

National Institutes of Health

6610 Rockledge Drive, MSC 6612

Bethesda, MD 20892–6612

866–284–4107 or 301–496–5717

www.niaid.nih.gov

National Institutes of Health

Bethesda, MD 20892

www.nih.gov

MedlinePlus

National Library of Medicine

8600 Rockville Pike

Bethesda, MD 20894

888–FIND–NLM (888–346–3656) or 301–594–5983

www.medlineplus.gov

Centers for Disease Control and Prevention

1600 Clifton Road

Atlanta, GA 30333

800–CDC–INFO (800–232–4636)

www.cdc.gov

National Center for Immunization and Respiratory Diseases

Centers for Disease Control and Prevention

1600 Clifton Road

Atlanta, GA 30333

800–CDC–INFO (800–232–4636)

www.cdc.gov/vaccines
41NIAID I UNDERSTANDING MICROBES

Food and Drug Administration 

5600 Fishers Lane 

Rockville, MD 20857–0001 

888–INFO–FDA (888–463–6332) 

www.fda.gov 

U.S. Department of Agriculture

Agricultural Research Service 

5601 Sunnyside Avenue

Beltsville, MD 20705–5134 

www.ars.usda.gov 

World Health Organization 

Avenue Appia 20 

1211 Geneva 27 

Switzerland

(00–41–22) 791–21–11 

www.who.int
42NIAID I UNDERSTANDING MICROBES

Credits

Cover CDC* Public Health Image Library

Facing Contents NIAID†

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Page 19 NIAID

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Page 28 © BigStockPhoto

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*Centers for Disease Control and Prevention (CDC)

†

National Institute of Allergy and Infectious Diseases (NIAID)
43NIAID I UNDERSTANDING MICROBES
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

National Institutes of Health

National Institute of Allergy 

and Infectious Diseases

NIH Publication No. 09-4914

September 2009

www.niaid.nih.gov