Chapter 7
Human host
(Entity)

Infectious diseases and their prevention
and protection

Onset ofinfectious diseases

Learning objectives
You will be able to gain proper understanding and explain:

Source of infection(Pathogen)- The three major factors for the establishment of infection and the definition of infectious diseases.
- Common infectious diseases in Cambodia and their causes, symptoms, and treatment.
- Common infectious diseases in Cambodia and workable preventive measures by taking into

Infection route(Hands, etc.)

Figure 7.1 Three major factors for establishment of infection: agent, infection route, and hostaccount the three major factors for the establishment of infection.
- Discrimination and prejudices that may occur with an infectious disease, and then develop ways
to prevent them.

2) Pathogens2,3- The functions of how the skin and mucous membranes as well as perspiration, tears, and saliva
work to prevent foreign substances from invading the body from outside.
- The types of defense mechanisms your body has to eliminate foreign substances that have invaded
the body.

Pathogens in general are small organisms that cause diseases. As shown in Table 7.1, pathogens may be
divided into different categories depending on their size and structure, including viruses, bacteria,
fungi, and parasites, each with different characteristics and causing different infectious diseases. Other
transmissible diseases include prion diseases, which are caused by abnormal proteins called prions.

This chapter illustrates infectious diseases, which represent important disease control issues in countries
around the world. Specifically, we will consider basic knowledge of the pathogenic mechanisms ofinfection, how biological defenses and immune systems protect our health against infection withpathogens, characteristics of major infectious diseases, and measures to prevent infectious diseases.

Table 7.1 Major pathogen categories and their characteristics3–5

Pathogencategories

Sexually transmitted diseases will be discussed in Chapter 11.
Viruses

1. Definition of infectious diseases and three major
factors for the establishment of infection1) Definition of infectious diseases1

Biologicalclassification

Characteristics

A virus consists of nucleic acid
(DNA or RNA) and protein.
Nonliving things
Viruses are incapable of selfMeasure approx.
replication; They bind to living
1–10 nmcells and get taken up, then use
those to copy themselves.

Influenza virus, Ebola virus, coronavirus,
human immunodeficiency virus, hepatitis
virus, rabies virus, measles virus, rubellavirus, chickenpox virus, rotavirus,
norovirus, adenovirus

Bacteria do not have a nuclear
membrane but do have a cell
membrane.

Staphylococci, Streptococcus viridans,
Clostridium tetani, Klebsiellapneumoniae, Chlamydia psittaci,
Legionella pneumophila, Clostridium
botulinum, campylobacter, Escherichia
coli, Vibrio cholerae, Shigelladysenteriae, Salmonella enterica, Vibrio
parahaemolyticus

Bacteria

Prokaryotic
Approx. 1–10
μm

Fungi

Fungi include yeasts, molds,
Eukaryotic
Approx. 10–100 and mushrooms.
μm

Infection refers to the invasion and growth of a pathogen inside or on the surface of the body of a host.
A pathogen is a cause of infectious diseases, such as bacteria, viruses, and parasites that can produce
diseases. A host is an entity such as a person or animal that is infected by bacteria or viruses. A disorderin vital functions of a host that results from infection is called a symptom, while the manifestation ofsymptoms or onset of disease refers to a state in which symptoms are present. Diseases that are causedin this manner are collectively referred to as infectious diseases.
How infection occurs can vary depending on the time, place, and individual characteristics.
Infection requires three factors to be established, namely, pathogens, infection routes, and hostsusceptibility (Figure 7.1).

112

Parasites(protozoa)

ExamplesEukaryotic

Protozoa are a type of parasitethat are single-celled.

Candida, aspergillus

Malaria parasite, liver fluke,
schistosome, Trichomonas vaginalis,
Toxoplasma gondii

Note 1: The pathogens underlined are major causes of food poisoning.
Note 2: Units: 1 nm (nanometer) = 10-9 m; 1 μm (micrometer) = 10-6 m.
Note 3: The Pathogens in italics are academic names.

Chapter 7

Infectious diseases and their prevention and protection

113

Those that affect cows include bovine spongiform degenerative encephalopathy (also known as Mad

1) Physical defense

Cow Disease), while human prion diseases include Creutzfeldt-Jakob disease (CJD).
A living body is in contact with the outside world through the skin and mucous membranes. The skin3) Infection routes: vertical and horizontal transmissions

and mucous membranes have physical defense systems that prevent foreign substances from invading
the body.
The stratum corneum, which is found on the side of the skin that comes in contact with the outside

There are roughly two routes of infection, or ways through which a pathogen can enter a living organism:

world (i.e. the outermost layer), functions as a barrier. The stratum corneum barrier consists of corneocytes

vertical transmission and horizontal transmission. Vertical transmission is the transmission ofand stratum corneum intercellular lipids that fill the space between the cells. It functions to protect thepathogens to a fetus or newborn that occurs during pregnancy or childbirth. It is commonly known asbody against the invasion of foreign substances and plays the role of retaining water in the body.

mother-to-child transmission. This can occur across the placenta, through the birth canal, or by breast

The digestive system, including the mouth, esophagus, and digestive tract, and the respiratory
system, including the trachea, are separated from the outside world by membranes. The lumen of themilk.
Horizontal transmission is the spread of pathogens from the source of infection, such as people ordigestive system is covered with membranes, which work to protect against the invasion of pathogens

objects, to others. It may take forms of contagious transmission (oral, mucosal, or sexual transmission),and other foreign substances. Living organisms have approximately 60–70% of their immune system indroplet transmission, airborne transmission, and fomite (vector) transmission. Fomite transmission

the digestive system, which protects the living organisms against infection with pathogens.

includes infection through blood, secretions, ingestion, and vectors. According to WHO6, over a million

The respiratory tract is lined with hair-like projections called cilia, which move the mucous layer

people die every year from infection via vectors such as mosquitoes, freshwater aquatic snails, ticks, andupward to eliminate foreign substances. The mucous layer captures foreign substances so as not to allow

fleas. Waterborne transmission is a form of oral transmission and is a leading cause of diarrhea due toany pathogens or atmospheric particles to reach the lungs during breathing.

infection to the digestive tract, which can cause explosive outbreaks regardless of age and sex, particularly
in developing countries7. It is responsible for 2.2 million deaths globally every year, and most of thevictims are children in developing countries7.
1 Physical and chemical

defense4) Host susceptibility

1

When the same pathogens enter into living organisms, they may or may not develop symptoms. This isdue to the differences in the amounts of invading pathogens and host resistance, or activities of theimmune system. When a host is susceptible to pathogens, the activities of their immune system are

2 Natural immunity (non-specificimmunity) defense

3 Acquired immunity (specificimmunity) defense

3

2

Pathogencompromised, and the host may develop symptoms if infected. On the other hand, when the host is not
susceptible to the pathogens, which means that he/she has strong immune resistance, the immune systemeliminates the pathogens that have invaded his/her body and prevents him/her from developing
symptoms.

2. Biological defense mechanisms

8

Living organisms have biological defense mechanisms, which help them protect themselves against
pathogens and other foreign substances (i.e. substances that are not normal constituents of the body)Invasion of body

Onset ofdiseaseCopyright © JAPAN BLOOD PRODUCTS ASSOCIATION. All Rights Reserved.
Translated with permission.

Figure 7.2 Three defense mechanisms to protect the body

(Figure 7.2).

114

Chapter 7

Infectious diseases and their prevention and protection

1152) Chemical defenseline of defense”; also called the adaptive immune system). They work in stages: first the natural immune
system, followed by the acquired immune system. The natural immune system represents primitive

Chemical defense is included as a part of the physical defense mechanism. Perspiration, tears, and saliva

defense mechanisms that humans are born with, and it works to eliminate invading pathogens throughcontain an enzyme called lysozyme, which breaks down cells of pathogens (“bacteriolytic activity”).

phagocytosis. The defense provided by the natural immune system has limits, however, which is why

Sebum and gastric acid have acidic pH conditions. This helps suppress the growth of pathogenic bacteria,

the acquired immune system plays an important role.
Such immune responses involve a number of cells and organs, and white blood cells play a central

which are not highly resistant to acids.

role. White blood cells come from their precursor cells known as hematopoietic stem cells that arepresent in the bone marrow. When matured, they become cells such as granulocytes (neutrophils,3) Immune systemeosinophils, and basophils), monocytes (which leave, or “migrate” from, the bloodstream into tissues
that are invaded by foreign substances, and differentiate into dendritic cells and macrophages),If foreign substances such as pathogens have broken through the physical and chemical defense systems

lymphocytes (B cells, T cells — helper T cells and killer T cells —, and natural killer (NK) cells) (Table(the “first line of defense”), they are inside the living organism. The system that eliminates invading

7.2). Note that some other types of blood cells, such as platelets and red blood cells, also differentiate

pathogens that have entered the body is called the immune system. The immune system consists of thefrom hematopoietic stem cells. Of these types of white blood cells, those responsible for natural immunity

natural immune system (the “second line of defense”) and the acquired immune system (the “third

are neutrophils (which account for 40–70% of white blood cells), monocytes (dendritic cells andmacrophages; approx. 5% of white blood cells), and NK cells, while acquired immunity involves T

Table 7.2 Types and functions of key white blood cells involved in the immune system

Type of white blood cell

Major roles

Neutrophils

Phagocytic and bactericidal; cause
inflammation.

Dendritic cells (differentiatedfrom monocytes migrating fromthe bloodstream to the tissues)

Phagocytic; communicate antigen
information (“antigen
presentation”).

White blood cells

Type ofimmunity

Killer T cells (differentiated fromT cells which mature in thethymus gland)

NaturalLymphocytes
B cells (differentiate and maturein the spleen)

Natural killer (NK) cells

116

macrophages migrate from the bloodstream and gather in the invaded tissues to take these bacteria intoand “heterologous” or other components. Phagocytes digest and break down foreign substances thathave been taken up with cellular enzymes. This process is called the phagocytic mechanism.
In the case of virus infection, unlike bacterial infection, phagocytosis has limits on its own. This iswhere NK cells come in, directly attacking and destroying the virus-infected cells. NK cells, too,recognize autologous and heterologous components, and usually do not attack uninfected or normal
cells (see Immunological tolerance).

b. Inflammation
Acquired

When natural immunity provided by phagocytosis has caused substantial damage to the body (such as
mass cell deaths), macrophages recognize it and gather neutrophils in the bloodstream, causing a
localized inflammatory response. As the capillaries relax and dilate, the blood flow is increased locally,

Proliferate, become activated, anddifferentiate into antibodyproducing cells (plasma cells) to
produce antibodies that are specificto the antigen; present antigen;become memory cells to prepare
for invasion of the same antigen.
A potent killer; directly attack andeliminate virus-infected cells andautologous cancer cells.

When bacteria have entered into a living organism, cells such as   neutrophils, dendritic cells, andthe cells. Known as phagocytes, these cells recognize “autologous” or normal components of the body,Receive antigen information fromdendritic cells and become
activated, promoting proliferation
and activation of B cells, whichhave the same antigen information;some become memory cells to
prepare for invasion of the same
antigen.
Receive antigen information fromdendritic cells, and attack andeliminate infected cells; somebecome memory cells to prepare
for invasion of the same antigen.

responsible for the immune function that protects the body against parasitic infection.

(1) Natural immune systema. Phagocytosis

Macrophages (differentiated from Phagocytic and highly bactericidal;
cause inflammation; present
monocytes migrating from theantigen.
bloodstream to the tissues)
Helper T cells (differentiatedfrom T cells which mature in thethymus gland)

cells (helper T cells and killer T cells) and B cells. These are the typical immune cells. Eosinophils arewhich in some cases causes a plasma leakage, accompanied by tumefaction and rubor. Macrophages
release substances into the bloodstream that work on the hypothalamus of the brain (i.e. interleukins),
thereby increasing the body temperature (i.e. fever) and activating the immune cells. In other words, an
inflammatory response is proof that the immune system is given a boost.
Natural

Chapter 7

Infectious diseases and their prevention and protection117

(2) Acquired immunity
a. Communication from the natural to the acquired immune system (Figure 7.3)

In acquired immunity, upon invasion by the pathogens or other foreign substances that have the same

The acquired immune system is characterized by its specific activity against a specific kind of foreign

information as that is stored in memory cells, B cells promptly differentiate into antibody-producing

substance. An antigen is a substance that induces acquired immunity. In general, exogenous pathogens

cells (plasma cells) to produce antibodies. In general, they produce antibodies in a larger amount andare recognized as antigens. First, dendritic cells and macrophages, which work as a part of natural

more quickly than when the first infection occurred and eliminated the pathogens. As a result, an

immunity, digest and break down the foreign substances such as pathogens through phagocytosis. Then

individual may have relatively mild symptoms or may not manifest symptoms at all in a second or

they take up the foreign substances (antigens) and move to lymph nodes, where they present the antigen

subsequent infection.

information on the cell surface, and deliver the antigen information to T cells (helper T cells and killer
T cells), which work as a part of acquired immunity that follows. The mechanism in which antigen
information is transferred from the natural to the acquired immune system is called antigen presentation.

c. Secondary immune response(3) Immunological tolerance
Immunological tolerance is the body’s mechanism that prevents the immune system from wrongly
recognizing normal autologous cells and tissues as antigens and attacking them. On rare occasions, thisNatural immunity
Neutrophil

Macrophage

Acquired immunity
T cellAntigenpresentationum

normal autologous cells and tissues as antigens and attacking them. This is called autoimmune diseases,
which include type 1 diabetes and rheumatoid arthritis.

3. Prevention of infectious diseases

B cellPathogens

immunological mechanism fails to work properly, resulting in the immune system wrongly recognizing

Antibody

Figure 7.3 Antigen presentation from natural to acquired immune system

b. Cell-mediated and humoral immunity

To prevent infectious diseases, it is necessary to cut off the chain of three major factors for theestablishment of infection described earlier in 1. Specifically, it is necessary to eliminate pathogens(source control), block the routes of infection (control of infection routes), and boost the host resistanceto diminish susceptibility to pathogens (susceptibility control) (Figure 7.4). By cutting off one of thefactors of the three principles, an epidemic of infectious disease may be prevented after an outbreak. In

The acquired immune system has two mechanisms, namely cell-mediated immunity and humoral

addition, the practice of preventive behavior by individuals and improvements in the social system are

immunity. In cell-mediated immunity, upon recognizing antigen presentation by dendritic cells andrequired for the prevention of infectious diseases.

macrophages, helper T cells and killer T cells proliferate and become activated, and move to the infectedsite. There, killer T cells capture and destroy the infected cells in a specific manner. At the same time,
proliferated helper T cells activate neutrophils, macrophages, and B cells. Even after the elimination of
the antigen, helper T cells and killer T cells promote the differentiation of the activated B cells intomemory cells (i.e. memory B cells), which remain in the body for a long time to prepare for any

Keys to prevention measures
Cut off the sourceof infection

Cut off the routeof infection

Boost immunity

There are different ways
for sterilization anddisinfection dependingon the pathogens.

Keep skin/clothes clean; clean
house; maintain water system;
ensure hygienic distribution/storage
and refrigeration of foodstuffs.

Keep a balanced diet;appropriate level of exercise,rest and sleep; immunization.

subsequent infection. Meanwhile, some of the T cells become memory T cells (memory helper T cells
and memory killer T cells) to prepare for any subsequent infection.
In humoral immunity, first proliferated helper T cells activate the B cells that have recognized thesame antigen. Then the B cells differentiate into the antibody-producing cell (plasma cells), andproduce antibodies. Antibodies are made up of proteins called immunoglobulin, and they deactivate
the invading antigen by binding specifically to it (“antigen-antibody reaction”). The deactivatedantigen is eliminated through phagocytosis by neutrophils and macrophages. Once the antigen has
attenuated in the living body, normally the antibodies also disappear in two to three weeks, but asdescribed above, some of them remain in the body as memory cells to prepare for any subsequent
infection.
118

Figure 7.4 Keys to prevention measures to cut off the three major factors

for establishment of infection

Chapter 7

Infectious diseases and their prevention and protection

1191) What can individuals do?

the leadership of the WHO. For the purpose of ensuring the quality of the vaccine, a manual on
vaccine production was created and guidance given on its production, while a simplified technique

We will discuss measures an individual can take that are based on the three principles of infection.

for vaccination was developed. As a result, following the case of a young Somali man in 1977, noSource control measures may include proper management of food to prevent pathogens from growing,cases of smallpox have been reported. In 1980, the WHO announced the global eradication of theand disinfection and sterilization of pathogens. An important measure for the control of infection routes

disease. The eradication of smallpox demonstrates that it is essential for countries to work together

is hand washing. While the routes of infection vary among different pathogens, many of the pathogens

across national boundaries to control infectious diseases.

that affect humans tend to adhere first to a hand; as the hand touches mucosal membranes of the nose,
mouth, and eyes, the pathogens can be introduced into the body. Moreover, as a person’s hands carrying
pathogens touch many objects and other people touch such objects with their hands, the infection spreads.

2) What are schools expected to do?

Washing hands thoroughly with soap is an effective way to prevent infection that anyone can do. For
infectious diseases that are caused through droplets produced by coughing and sneezing, it is effective

Since schools are where children learn in large groups, they are prone to the prevalence of droplet

in reducing droplets by having infected individuals wear masks and maintain cough etiquette (e.g. covertransmission and contagious transmission. For this reason, it is necessary to implement infectious

your mouth and nose with a handkerchief or sleeve when you don’t have a mask), and ventilating indoor

disease control measures based on the three principles of infection prevention. Source control measures

spaces as well (see Chapter 4). As a susceptibility control measure, one can boost their immunity. For

include improvement of the school environment. Drinking water stations and toilets, in particular, should

people to protect themselves from developing an infectious disease, and to keep it from becoming serious

be kept clean. It is preferable to inspect drinking water routinely to ensure that it meets safety standards

if they do develop one, it is important for them to boost their immunity. Specifically, they should form a(See Chapter 4 for drinking water standards).

healthy lifestyle based on a balanced diet, adequate sleep, and an appropriate level of exercise (seeHealth education of pupils and students is important for the implementation of control measures for

Chapter 5). Vaccination (immunization) is another effective measure for preventing infectious diseases.

infection routes and susceptibility. Children should not only be given knowledge in class, but also learn

From the standpoint of preventing waterborne infectious diseases, changing from open urination andto be mindful of preventive behaviors against infection in their school life, such as those concerning thedefecation to the use of toilet facilities can prevent contamination of water lines, and thus help in theuse of toilets, hand washing after using the toilet, and cough etiquette. In Cambodia, prevention of

prevention of infectious diseases.

infectious diseases represents an important challenge. For Cambodians to learn basic knowledge on
infectious diseases and prevention skills at school helps them protect not only their own growth andColumn: Vaccines and immunization

Infectious diseases are caused by pathogens such as bacteria and viruses. However, the humanbody memorizes pathogens that once invaded and maintains a defense system that prevents thehealth, but the health of their families and communities, as well.

3) What is society expected to do?

onset of diseases or alleviates symptoms caused by the second infection and thereafter. That is theimmune system. Vaccination is a medical application of this mechanism. Vaccines contain weakened

Because infectious diseases spread from person to person, it is necessary to ensure not only the practiceor detoxifyed pathogens. Vaccination helps not only to protect you from infectious diseases, but alsoof preventive behaviors by individuals but also the improvement of social systems. Specifically, thishelps prevent infections to those around you. However, vaccines are attenuated or detoxified

means improving public health systems such as water and sawage systems in order to prevent infectious

pathogens, and the possibility of side reactions cannot be eliminated.

diseases, implementing quarantine in order to prevent infectious diseases from getting into the country,
and promoting immunization to give the population acquired immunity as prophylaxis. It is also

Column: Fighting smallpox

Humanity has fought many infectious diseases. The first infectious disease that humanity eradicatedimportant to develop a monitoring system to check on infection in the country, communicate appropriate
information to the people, and make use of the information gathered in studies for future measures to be
implemented.

was smallpox. Extremely contagious, smallpox was feared as a fatal disease since pre-Christiantimes. At the end of the 18th century, noticing that milkmaids did not catch smallpox while they did
catch cowpox, which is not fatal, an English physician named Edward Jenner developed immunization

120

Column: A new lifestyle(the smallpox vaccine). Although the introduction of the smallpox vaccine dramatically decreased

In the wake of the outbreaks of the novel coronavirus infectious disease (COVID-19) in 2020, a shift

smallpox deaths, the vaccine was potent only for a few days at room temperature and could not be

is called for to a new lifestyle, represented by mask wearing and maintaining a distance of at least 1

delivered to all corners of the world. In 1967, an initiative to eradicate smallpox was launched underm between one another. These measures not only change the way we live our daily lives, but will lead
Chapter 7

Infectious diseases and their prevention and protection121to changing how we work and how we live. We all feel reluctant to change the lifestyles we are

These symptoms disappear in about a week, and patients usually recover without sequelae. The severefamiliar with. It is not all a bad thing, however. As we have gained the habit of washing hands toform of dengue fever, namely dengue hemorrhagic fever, causes a patient to experience a sequence ofprevent COVID-19, morbidity due to influenza and food poisoning has also declined. As long as we

dengue fever symptoms, but before recovery, they experience bleeding, hypotensive shock, and multiple

are part of nature, there will always be infectious diseases caused by viruses. What we should aimorgan failure. It is noteworthy that severe symptoms tend to appear when a fever recedes and thefor is not to overcome viruses but to co-exist with them while minimizing the damage they cause.

temperature begins to return to normal. Pleural effusion and ascites occur at very high rates. Patients
may also experience enlargement of the liver, activation of complements, platelet depletion, and/or

4. Common infectious diseases in Cambodia

prolonged blood clotting. In addition, as the name “hemorrhagic fever” implies, 10–20% of the patients
experience nasal and/or gastrointestinal bleeding. Treatment for dengue fever is symptomatical.
For the prevention of infection, it is important to take measures to avoid mosquito bites. Specifically,

An outline of infectious diseases commonly seen in Cambodia is given below.

one shoud wear long-sleeved shirts and long pants, apply an insect deterrent, and sleep under a mosquito
net. It is also important for the purpose of prevention to make community-wide efforts to exterminate
mosquitoes and suppress breeding of mosquitoes (e.g. eliminating puddles, cutting grasses).

1) Dengue fever
According to a WHO report9, a total of 9,108 dengue cases with 14 deaths had been reported in Cambodia

Column: What are complements?

by September 2020. This number of cases is only 15% of the 61,198 cases in the same period last year.

Complements are a collective term for proteins that mediate immune response to eliminate antigens

Dengue fever is still one of the most serious infectious diseases in Cambodia, which has a tropical climate.

such as pathogens invading a living organism. They are so named because they complement the

Dengue fever is an infectious disease caused by the dengue virus and spread by the yellow feverfunction of antibodies.

mosquito (Aedes aegypti) and the tiger mosquito (Aedes albopictus). The dengue virus is of the family
Flaviviridae, and has four serotypes (DENV 1–4). The dengue virus forms a human-mosquito-humantransmission cycle (Figure 7.5). It is believed that most of the cases of dengue virus infection end up

2) Malaria

being inapparent or subclinical.
Malaria is an infectious disease that results from the transmission of a malaria parasite into the humanbody via female anopheles mosquitoes. After 1–4 weeks of incubation, a malaria parasite proliferates

Humanin the hepatic cells and red blood cells, and causes symptoms such as fever (which has cyclicity of
repeating febrile periods followed by afebrile ones), headache, joint pain, nausea, anemia, and spleenenlargement. Few patients develop all of the symptoms from the beginning; many follow atypicalcourses. There are four types of malaria that affect humans depending on the species of the parasites:
Mosquito

Mosquito

Plasmodium falciparum (P. falciparum), Plasmodium vivax (P. vivax), Plasmodium ovale (P. ovale), andPlasmodium malariae (P. malariae). Incubation periods, fever cycles, and treatment methods varydepending on the type of the parasites. Treatment is provided by antimalarial drugs; the choice of drug
depends on the type of the parasites. There is no immunization for malaria. For prevention, it is importantHuman

to take measures to avoid mosquito bites (see Dengue fever).

Figure 7.5 Transmission cycle of dengue virus

3) Tuberculosis
For those who do manifest symptoms, there are two types: a relatively mild form of dengue fever
and dengue hemorrhagic fever, which is a severe form. Dengue fever manifests as a sudden onset ofTuberculosis is an infectious disease caused by inhalation of tubercle bacilli. It is the leading cause offever 3–7 days into infection, often accompanied by a headache (especially orbital pain), muscle pain,

death linked to a single pathogen worldwide, followed by AIDS10. According to WHO statistics10,

and joint pain. It may also be accompanied by a loss of appetite, stomachache, or constipation. Three toapproximately 10 million (range: 9–11 million) patients contracted tuberculosis in 2018; the numbers

four days into infection, a red rash appears on the chest and trunk and then spreads to the limbs and face.

have remained constant for the last few years. While tuberculosis affects people regardless of sex and122

Chapter 7

Infectious diseases and their prevention and protection123

age, it was most prevalent in men aged ≥15 years, accounting for 57% of all the patients. It was estimated4) Rabiesthat women accounted for 32% and children of age <15 years, 11%. Geographically, most tuberculosis
cases were found in India (27%), China (9%), Indonesia (8%), the Philippines (6%), Pakistan (6%),

Rabies develops when a person is bitten or scratched by a dog, cat, bat or other wild animal that is

Nigeria (4%), Bangladesh (4%), and South Africa (3%); these eight countries accounted for 67% of theinfected with the rabies virus, and the virus invades the body through the wound. The duration ofworld’s patients. Cambodia is among the 30 countries with the highest tuberculosis burden, with 49,000

incubation after infection, while it may vary depending on the site of the bite, is usually 1–2 months.

11

patients and an incidence of active tuberculosis of 302 patients per 100,000 population . This is

Symptoms of a common cold may develop, including fever, headache, lethargy, muscle pain, tiredness,

approximately 2.3 times the average among the WHO Member States of 132 patients per 100,000

poor appetite, nausea/vomiting, sore throat, and dry cough. It may be accompanied by pain at the bite

population, which warrants tuberculosis control measures12.

site and loss of sensation or muscle spasms around the site. Symptoms of encephalitis appear, starting

While tuberculosis is a systemic infectious disease, pulmonary tuberculosis, which causes lungwith excitement, anxiety, and mania, followed by delirium, hallucination, aggressiveness, and muscle

lesions, accounts for as much as approximately 80% of the cases. As the tubercle bacilli start to movespasms such as hydrophobic fits. Patients eventually progress to coma and die of respiratory arrest.

hematogenously in the body, lesions form in a number of organs, including the pleura, lymph nodes,

Rabies, once developed, has a mortality rate of almost 100%. Due to its long incubation period, its onset

kidneys, and bones, and symptoms appear at each site. If the tubercle bacilli enter the lungs through the

can be averted by continuous vaccination starting immediately after any events that may cause infection

respiratory tract and cause a small initial focus of infection, it is regarded that initial infection is

to happen. Since rabies is a zoonosis, wild dog management and immunization of domestic dogs are

established (first infection). However, many such patients do not manifest symptoms and become

important for prevention.

carriers. They may spend years as carriers with the tubercle bacilli being established in the lung alveoli
and may manifest symptoms only after several years to decades when they become seniors or become
ill and have weaker immunity (second infection). Children often become infected through household

Column: Zoonosis

transmission, in which most cases are primary tuberculosis (first infection). Common treatment for

A zoonosis is a disease that can be transmitted from animals to humans and vice versa. Zoonotic

tuberculosis is a combination of anti-tuberculosis drugs. There are concerns over the emergence ofdiseases make up for approximately 50% of all infectious diseases, and include a number of diseases

multi-drug-resistant tuberculosis, however, and WHO

13

recommends Directly Observed Treatment,

other than rabies, such as avian influenza, SARS, clonorchiasis, and plague. For this reason, theShort-Course Chemotherapy (DOTS), where healthcare professionals, health nurses, and family

concept of One Health, which means that human health and animal health are interdependent, ismembers directly check on patients’ compliance to help them avoid missing doses. Immunization is

advocated primarily among veterinarians.

14

effective, and Cambodia recommends immunization of infants within the first 12 months after birth .
Column: Carriers (asymptomatic carriers)

5) Parasitic infections

Some people may be infected with a virus and carry it inside their bodies, but do not manifest

There are many parasitic infectious diseases found in Cambodia. Of these, those that are particularly

symptoms. This is a state where the strength of the pathogens and the person’s immunity are wellcommon, namely trematode and nematode infections, are discussed in this chapter.

balanced, and which suppresses manifestation of symptoms. Even though they do not show any
symptoms, such people still shed bacteria and viruses, and thus risk infecting others around them.

(1) Trematode infections

Such people are called bacterial or virus carriers for bacterial or virus infection, respectively, andTrematodes bear two suckers, one at the end of the body and the other on the ventral surface, and are flat

those who do not show particular symptoms are called asymptomatic carriers. In the spread of aand symmetric. For them to grow, trematodes always require intermediate hosts, in which larvae

new coronavirus, tuberculosis, and HIV, such carriers play a significant role, and it is important for

multiply by division. They grow inside a first and second intermediate hosts, and eventually become

people to exercise preventive measures and get tested even if they are not aware of any symptoms.

adult in the definitive host. Common trematodes include schistosomes, which are transmitted via a
percutaneous route and live in the host’s blood vessels, and trematodes that settle in organs through oraltransmission. Depending on the type of trematode, various symptoms develop. Particularly problematicin Asia are four species of foodborne trematodiases (Table 7.3).
Trematodes infect freshwater aquatic snails as a first intermediate host. A second intermediate host
may vary depending on the type of trematode; freshwater fish for clonorchiasis and opisthorchiasis,and crustaceans for paragonimiasis, while fascioliasis does not require a second intermediate host. The
definitive hosts are always mammals. Humans become infected by trematodes by eating raw fish,

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Chapter 7

Infectious diseases and their prevention and protection125

Table 7.3 Epidemiological characteristics of foodborne trematodiases

DiseaseAcquired throughconsumption of

Infectious agent

Food poisoning is a foodborne illness caused by intake of naturally occurring toxins, chemical
substances, or pathogen found in food. Adverse health effects of exposure to naturally occurring toxins

Natural definitive hosts of
the infection

may be caused by an intake of wild mushrooms or plants, poisonous fish such as pufferfish, or molds.

Clonorchiasis

Clonorchis sinensis

Fish

Dogs and other fish-eating
carnivores

Opisthorchiasis

Opisthorchis viverrini, O.
felineus

Fish

Cats and other fish-eating
carnivores

Fascioliasis

Fasciola hepatica, F.
gigantica

Aquatic vegetables

Sheep, cattle and otherherbivores

Crustaceans (crabs andcrayfish)Cats, dogs and othercrustacean-eating carnivores

Paragonimiasis Paragonimus spp.
Note: Infectious agents in italics are academic name.

Food poisoning is commonly caused by pathogens. Other causes include chemicals, such as metal
poisoning caused by copper pots or dishes for eating or lead in canned food, and histamine poisoning
caused by raw fish or processed fish.

(1) Food poisoning caused by pathogens

15

Source: World Health Organization

A common form of food poisoning is that caused by pathogens, which can be roughly divided into
bacterial food poisoning and viral food poisoning. In general, bacterial food poisoning is common in
an environment suitable for bacterial growth, namely when it is warm and humid. Viral food poisoning,on the other hand, tends to be more common when it is cold with low humidity.

crustaceans, or vegetables contaminated with larvae. According to estimations by the WHO Foodborne
Disease Burden Epidemiology Reference Group (FERG) (2015)15, the leading four species of foodborne

a. Bacterial food poisoning

trematodes cause physical disorders in an estimated total of 200,000 people and more than 7,000 deaths

Common forms of bacterial food poisoning include cholera and dysentery, as well as infection withper year. For treatment, patients take an anthelminthic (such as praziquantel) orally.

campylobacter living in the intestines of poultry. While many types of food poisoning occur as a result(2) Nematode infectionsof infection with 100,000 to 1,000,000 viable bacteria, dysentery and Campylobacteriosis require amuch smaller number of bacteria for infection. In many cases, predominant symptoms of food poisoning

Nematodes have a cylindrical body. There are approximately 500,000 species known, of which some 50

are vomiting, diarrhea, and fever, but some may be accompanied by a headache or neurologicalparasitize human bodies. They do not always require an intermediate host, and some may repeat cycles

symptoms. Particular caution is required with infection with verotoxin-producing Enterohemorrhagicof growth and reproduction inside the definitive host. Many nematodes are transmitted orally, but someEscherichia coli (EHEC), O-157, O-26, O-111, and O-128. In addition to a high fever and severe

are transmitted via a percutaneous route. In this chapter, we discuss the roundworm (Ascaris

diarrhea, they may cause hemorrhagic enteritis and hemolytic uremic syndrome, which may progress tolumbricoides) and the human pinworm (Enterobius vermicularis). With ascariasis, a person becomes

kidney failure or encephalopathy resulting in death.

infected through ingestion of the worm’s eggs with food. They typically show digestive symptoms, andmay cause acute abdominal conditions in the case of erratic parasitism in the biliary tree. Enterobiasis isb. Viral food poisoning

caused when a person happens to take stools or food contaminated with stools in the mouth, or eats with

Viral food poisoning occurs as a result of intake of a virus carried by shellfish or virus-contaminateda hand that has touched a person or object contaminated with stools. It is particularly prevalent inwater or food. Common forms include norovirus and rotavirus infections. While bacterial foodchildren. The worms lay eggs around the anus, which causes itchiness around the area. Infection can also

poisoning is caused by bacteria that are attached to the surface of foodstuff and grow in the food, viruses

be transmitted orally via fingers that have scratched the area around the anus. It may lead to urethritis,

do not grow in food but inside the body or cells of a host.

vaginitis, or hepatitis. For treatment, patients take anthelminthic (e.g., pyrantel pamoate) orally for both
the roundworm and human pinworm.

(2) Prevention of food poisoning
A key to the prevention of food poisoning is the exercise of the “three principles of food poisoning

6) Enteric infections

prevention,” which ensures that we “do not allow pathogens to come in contact with food,” “do notallow pathogens to multiple,” and “destroy all pathogens.” Heat kills many of the bacteria responsible
for food poisoning. Bacteria such as campylobacter die when the contaminated food is heated at a center

Enteric infections refer to a group of diseases characterized by the manifestation of symptoms such astemperature of 75°C for 3 minutes or longer. Meanwhile, toxin-producing bacteria are more heat

diarrhea, vomiting, and stomachache caused by bacteria, viruses, and protozoans. Since there are a large

resistant. A toxin from Staphylococcus aureus will not die even after heating at 100°C for 30 minutes,

number of causative pathogens and their symptoms are similar, their diagnoses are challenging. It isbut will be killed at 200°C for 30 minutes or longer. It is therefore common to use antibacterial agents.

however important to identify the cause as precisely as possible because the treatment varies depending

Many pathogenic bacteria multiply by binary fission every 20–30 minutes. One E. coli bacterium,on the type of pathogen. In this chapter, we discuss the most important type of enteric infection, namely

for instance, becomes eight after an hour, and 1022 after 24 hours. When food contaminated withfood poisoning.

pathogenic bacteria is left at room temperature, the bacteria multiply in a short period of time. They do

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Chapter 7

Infectious diseases and their prevention and protection127not die even in the refrigerator; they simply multiply at a slower rate. While antibacterial agents areis a growing demand for enhanced public health systems based on international cooperation, for example,

effective against bacterial growth, they are not against viruses. Vaccines are effective against viruses, yet

supported by WHO.

not all viruses have vaccines, and everyday exercise of preventive measures is important.
It is possible to prevent food poisoning by means of disinfection with 70–80% alcohol and invert
soap. In the case of norovirus, however, alcohol is not highly effective, and disinfection with sodium
hypochlorite is necessary.

Other than those mentioned above, challenges we are facing include the rise of multidrug-resistantorganisms, which are pathogens resistant to newly developed drugs, and animal-derived infectious
diseases due to increases in imported pets and changes in how people interact with them.
Furthermore, infectious disease control measures always face discrimination and prejudice.

Even when causative bacteria or viruses for food poisoning find their way into the body, how

Misconception, fear, and anxieties concerning an infectious disease can give rise to discrimination. Withsymptoms manifest can vary depending on the immune strength of the person infected. Because evenleprosy and HIV infection (AIDS), there were cases where national governments implemented control

asymptomatic carriers can spread infection through their hands or bodily waste, hand washing ispolicies such as isolation of patients, even though the risk of transmission was low in everyday settings.

extremely important.

Preventing infectious diseases is important, yet it is necessary to take into account the human rights of
the infected when implementing measures, so as not to let them become a target of discrimination.

Column: What is invert soap?

Although called “soap,” invert soap has properties that are opposite to those of regular soap. It is
not capable of removing dirt or stain but has bactericidal and sterilizing properties to destroy the

Column: Preventing discrimination and prejudice from being induced by infectious

diseases

cells of molds and bacteria. It is not effective against viruses, which do not have cells. It is used forYear 2020 saw a global outbreak of novel coronavirus infectious disease (COVID-19). COVID-19

sterilizing fingers and dishes for eating, laundry, and cleaning. To sterilize fingers, one should firstis an emerging infectious disease, and we are faced with the challenge of responding to infectionwash hands thoroughly with regular soap to remove any dirt, then disinfect with invert soap.

prevention. In the course of our tackling this crisis, prejudice against infected people occurred. This
was nothing new. As the cases of AIDS and Ebola hemorrhagic fever are still fresh in our minds,
infectious diseases and pandemics have historically induced prejudice and discrimination. Behind

5. New challenges with infectious diseases

such discrimination are people’s anxieties over the illness. Prejudice and discrimination pose apublic health threat. They not only cause damage to physical and mental health and welfare of theThroughout human history, we have fought many infectious diseases, and successfully overcome somepeople who are subjected to prejudice, but they also make it difficult to contain the infectious

by improving healthcare standards and public health. On the other hand, we are facing new challenges.

disease. WHO recommends that governments, citizens, media, key influencers and communities

16

One example is emerging infectious diseases. WHO defines emerging infectious diseases as “thoseshould do their part to prevent prejudice and discrimination from spreading20. Steps suggested todue to newly identified and previously unknown infections which cause public health problems either

achieving this are: “Spread the facts”; “Engage social influencers”; “Amplify the voices of people

locally or internationally.” While not all emerging infectious diseases are fatal, their causes and routes

with lived experience with the coronavirus”; “Portray diversity of ethnicity in public informationof transmission are unknown at the beginning, which can cause a quick spread of infection. One of thematerials”; “Promote ethical journalism”; and “Correct myths, rumors, and stereotypes thatcauses behind these diseases is the increasing proximity between wild animal and human populations byencourage purejudice”21.

such activities as forest development. As there are more opportunities for humans to enter wildlife
habitats, humans may contact viruses carried by wild animals, or a virus that should originally infectonly animals may undergo the mixing of other viral genomes inside the cells of the animal and mutate
into a new type of virus that may prove extremely pathogenic when transmitted to humans. Because it
takes time to develop a new vaccine or therapeutic drug, emerging infectious diseases can pose challenges
on the prevention and treatment front.

Exercises for further thought and research[7-1] Do research on the laws and measures concerning infectious diseases and those concerning
hygiene management in Cambodia and give a presentation about them.

A second example is pandemics. A pandemic is defined as an epidemic of an infectious disease[7-2] Choose one infectious disease that is common in Cambodia and summarize the measures against

occurring across different countries and regions. With globalization comes greater movement of people,

it. In doing so, list up measures by the following categories: the three principles of infectious

and this can cause an infectious disease in one area to rapidly spread to multiple countries and regions,diseases; what individuals can do; what schools can do; and what society should do.

resulting in a global outbreak. WHO declared the H1N1 pandemic in 2009

17,18

, and the COVID-19

19

pandemic in March, 2020 . Because it is not possible to prevent a pandemic from occurring throughindividual countries’ efforts alone in implementing thorough infectious disease control measures, there
128[7-3] Create a poster promoting the prevention of food poisoning. Discuss as to where it should be
posted for it to be effective, keeping your target audience in mind.
[7-4] Discuss in groups what brings about discrimination and prejudice related to an infectious disease.
Chapter 7

Infectious diseases and their prevention and protection

129Plan a lesson on the theme of infectious diseases and human rights.
[7-5] Propose a new lifestyle for a new era of people living with the novel coronavirus in Cambodia.

Chapter 8

Eye and dental health
Learning objectives
You will be able to gain proper understanding and explain:- How we can see an object and why refractive errors occur, and explain how we can prevent eye
diseases.

References- The function of a tooth, the causes of dental caries, gingivitis, and periodontal disease, and explain
how we can prevent them.

1Center Hospital of the National Center for Global Health and Medicine, Antimicrobial Resistance Clinical Reference Center. Basics

2

Tokyo Food Hygiene Association. A guide to prevention of food poisoning as a threat to your family health. 2019. https://www.

This chapter explains the mechanisms of the eyes and the teeth, diseases related to them, and prevention

fukushihoken.metro.tokyo.lg.jp/shokuhin///pamphlet2/files/r1july_guide.pdf

of those diseases. Eyes and teeth, which are organs familiar to children in that they can easily observeof infectious diseases: What is an infectious disease? http://amr.ncgm.go.jp/general/1-1-1.html

3

Kitasato H, et al. Viruses and bacteria: Illustrated. Tokyo: Gijutsu Hyoronsha, 2020.

4

Hatanaka M and Shimada J, Ed. Microbiology. Tokyo: Bunkodo, 2004.

5

Yamazaki S, et al, Ed. Companion guide to infectious disease prevention. Tokyo: Japan Public Health Association, 2001.

make them realize the outcomes of health learning easily. Therefore, eye and dental health is one of the

6

World Health Organization. Vector-borne diseases. https://www.who.int/en/news-room/fact-sheets/detail/vector-borne-diseases

teaching and learning materials of school health familiar to all members of a class.

7

World Health Organization. Water sanitation hygiene. https://www.who.int/water_sanitation_health/diseases-risks/diseases/

them, provide them with opportunities to experience, for example, preventive measures in practice anddiarrhoea/en/
8

Byoki ga mieru: Vol. 6, Immunology, Collagen Diseases and Infectious Diseases (2nd ed). Medic Media, 2018.

9

WHO Western Pacific region. Update on the Dengue situation in the Western Pacific Region. Dengue Situation Update Number 610.
2020. https://www.who.int/docs/default-source/wpro---documents/emergency/surveillance/dengue/dengue-20201105.pdf?sfvrsn=
fc80101d_42

10 World Health Organization. Global Tuberculosis Report 2019: Executive Summary. https://www.who.int/tb/publications/global_
report/tb19_Exec_Sum_12Nov2019.pdf?ua=1
11 World Health Organization. Global Tuberculosis Report 2019: Annex2 Country Profile For 30High TB Burden Countries. https://www.who.int/tb/publications/global_report/tb19_Report_country_profiles_15October2019.pdf?ua=1
12 World Health Organization. Global Tuberculosis Report 2019: Annex 3 Regional and Global Profile. https://www.who.int/tb/
publications/global_report/tb19_Report_regional_global_15October2019.pdf?ua=1
13 World Health Organization. Tuberculosis: What is DOTS (Directly Observed Treatment, Short Course). http://origin.searo.who.int/
tb/topics/what_dots/en/
14 Cambodia Demographic and Health Survey 2014, 14. Child Health-Immunization of Children, P.152. National Institute of Statistics(Cambodia), Directorate General for Health (Cambodia) and The DHS Program (USA), 2015. https://dhsprogram.com/pubs/pdf/
FR312/FR312.pdf
15 World Health Organization. Foodborne trematode infection. 2020. https://www.who.int/news-room/fact-sheets/detail/foodbornetrematode-infections
16 World Health Organization. Emerging Infections Diseases. World Health Day 97.1. https://www.who.int/docstore/world-health-day/en/documents1997/whd01.pdf
17 Doshi P. The elusive definition of pandemic influenza. Bulletin of the World Health Organization 2011;89:532-538. doi: 10.2471/
BLT.11.086173. https://www.who.int/bulletin/volumes/89/7/11-086173/en/
18 WHO declares influenza A(H1N1) pandemic. https://www.youtube.com/watch?v=10Nfk0zcTAk
19 Coronavirus outbreak: WHO declares COVID-19 a global pandemic. https://www.youtube.com/watch?v=xKPWngYf2Wk

1. Eye health1) Importance of eye health
Eye and dental diseases signiﬁcantly affect our health not only during school age but also in adulthood.
Loss of sight in particular is a serious issue. Cambodia implemented the national strategic plan to prevent
loss of sight (2008–2015) with a focus on the National Program for Eye Health, and achieved the goalof making the rate of blindness 0.5% or lower.1 Nevertheless, an estimated 28,800 people still sufferfrom loss of sight or visual impairment. The leading cause of loss of sight and visual impairment iscataracts (approx. 19,000 people), but they can also be caused by leaving refractive errors that occur
during school age untreated. It is said that 80–90% of these causes are preventable or treatable.2
Meanwhile, about 20% of children have missed a class due to a toothache over the last 12 months,
indicating that dental health is an issue that should be given priority for school-age children.3 The school
age is a period in which milk teeth (deciduous teeth) are replaced by permanent teeth, and leaving dental
caries or periodontal disease untreated during this period can invite exacerbation later and result in lossof teeth in adulthood.
Therefore, it is very important to learn eye and dental health in school health classes during schoolage.

20 IFRC, UNICEF, WHO. Social Stigma associated with COVID-19: A guide to preventing and addressing social stigma. https://www.
who.int/docs/default-source/coronaviruse/covid19-stigma-guide.pdf
21 American Psychological Association. Combating bias and stigma related to COVID-19: How to stop the xenophobia that’s spreadingalong with the coronavirus. https://www.apa.org/topics/covid-19-bias

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Eye and dental health131