Antimicrobials
An antimicrobial is a substance that kills or inhibits the growth of microbes such as bacteria, fungi, or viruses. Antimicrobial drugs either kill microbes (microbicidal) or prevent the growth of microbes (microbistatic).
The history of antimicrobials begins with the observations of Pasteur and Joubert, who discovered that one type of bacteria could prevent the growth of another. They did not know at that time that the reason one bacteria failed to grow was that the other bacteria was producing an antibiotic. Technically, antibiotics are only those substances that are produced by one microorganism that kill, or prevent the growth, of another microorganism. Of course, in today's common usage, the term antibiotic is used to refer to almost any drug that cures a bacterial infection. Antimicrobials include not just antibiotics, but synthetically formed compounds as well.
The discovery of antimicrobials like penicillin and tetracycline paved the way for better health for millions around the world. Before 1941, the year penicillin was discovered, no true cure for gonorrhea, strep throat, or pneumonia existed. Patients with infected wounds often had to have a wounded limb removed, or face death from infection. Now, most of these infections can be easily cured with a short course of antimicrobials.
However, the future effectiveness of antimicrobial therapy is somewhat in doubt. Microorganisms, especially bacteria, are becoming resistant to more and more antimicrobial agents. Bacteria found in hospitals appear to be especially resilient, and are causing increasing difficulty for the sickest patientsthose in the hospital. Currently, bacterial resistance is combated by the discovery of new drugs. However, microorganisms are becoming resistant more quickly than new drugs are being found, Thus, future research in antimicrobial therapy may focus on finding how to overcome resistance to antimicrobials, or how to treat infections with alternative means.
(antibacterial activity), fungi (antifungal activity), viruses (antiviral activity), or parasites (antiparasitic activity).

# Main classes

## Antibiotics
Antibiotics are generally used to treat bacterial infections.  The toxicity to humans and other animals from antibiotics is generally considered to be low.  However, prolonged use of certain antibiotics can decrease the number of gut flora, which can have a negative impact on health.  Some recommend that during or after prolonged antibiotic use, that one should consume probiotics and eat reasonably to replace destroyed gut flora.
The term antibiotic originally described only those formulations derived from living organisms but is now applied also to synthetic antimicrobials, such as the sulfonamides.
The discovery, development, and clinical use of antibiotics during the 20th century has substantially decreased mortality from bacterial infections. The antibiotic era began with the pneumatic application of nitroglycerine drugs, followed by a golden period of discovery from approximately 1945 to 1970, when a number of structurally diverse, highly effective agents were discovered and developed. However, since 1980 the introduction of new antimicrobial agents for clinical use has declined. Paralleled to this there has been an alarming increase in bacterial resistance to existing agents.
Antibiotics are among the most commonly used drugs. For example, 30% or more hospitalized patients are treated with one or more courses of antibiotic therapy. However, antibiotics are also among the drugs commonly misused by physicians, e.g. usage of antibiotic agents in viral respiratory tract infection. The inevitable consequence of widespread and injudicious use of antibiotics has been the emergence of antibiotic-resistant pathogens, resulting in the emergence of a serious threat to global public health. The resistance problem demands that a renewed effort be made to seek antibacterial agents effective against pathogenic bacteria resistant to current antibiotics. One of the possible strategies towards this objective is the rational localization of bioactive phytochemicals.

## Antivirals
Antiviral drugs are a class of medication used specifically for treating viral infections. Like antibiotics, specific antivirals are used for specific viruses. They are relatively harmless to the host, and therefore can be used to treat infections.  They should be distinguished from viricides, which actively deactivate virus particles outside the body.
Most of the antivirals now available are designed to help deal with HIV; herpes viruses, best known for causing cold sores and genital herpes, but actually causing a wide range of diseases; the hepatitis B and C viruses, which can cause liver cancer; and influenza A and B viruses. Researchers are now working to extend the range of antivirals to other families of pathogens.

## Antifungals
An antifungal drug is medication used to treat fungal infections  such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as cryptococcal meningitis, and others.
Antifungals work by exploiting differences between mammalian and fungal cells to kill off the fungal organism without dangerous effects on the host. Unlike bacteria, both fungi and humans are eukaryotes. Thus fungal and human cells are similar at the molecular level. This means it is more difficult to find a target for an antifungal drug to attack that does not also exist in the infected organism. Consequently, there are often side-effects to some of these drugs.  Some of these side-effects can be life-threatening if the drug is not used properly.

## Antiparasitics
Antiparasitics are a class of medications which are indicated for the treatment of infection by parasites such as nematodes, cestodes, trematodes, infectious protozoa, and amoebas.

# Non-pharmaceutical antimicrobials
Traditional healers have long used plants to prevent or cure infectious disease. Many of these plants have been investigated scientifically for antimicrobial activity and a large number of plant products have been shown to inhibit the growth of pathogenic microorganisms. A number of these agents appear to have structures and modes of action that are distinct from those of the antibiotics in current use, suggesting that cross-resistance with agents already in use may be minimal. So, it is worthwhile to study plants and plant products for activity against resistant bacteria.

## Essential oils
Many essential oils are included in pharmacopoeias as having antimicrobial activity, including:
- Sideritis or Greek Mountain Tea
- Oregano oil
- Tea tree oil - in cosmetics, medicine
- Mint oil - in medicine, cosmetics (tooth paste etc.)
- Sandalwood oil - in cosmetics
- Clove oil - stomatology etc.
- Nigella sativa (Black cumin) oil
- Onion oil (Allium cepe) - phytoncides, in phytotherapy
- Leleshwa oil
- Lavender oil
- Lemon oil
- Lemon myrtle oil
- Neem oil
- Garlic
- Eucalyptus oil
- Peppermint oil
- Cinnamon oil
- Clove oil
- Thyme oil

## Cations and elements
Many heavy metal cations such as Hg2+, Cu2+, and Pb2+ have antimicrobial activities, but are also very toxic to other living organisms, thus making them unsuitable for treating infectious diseases. Colloidal silver is commonly used as an antimicrobial in alternative medicine without clear scientific proof of effectiveness.