Most disease-causing organisms, or pathogens, are too small to be seen without a microscope. Some (e.g., most viruses) are even too small to be visible under a light microscope and must be viewed with the more powerful electron microscope. Because of their microscopic size, these minute organisms often are referred to as microbes or microorganisms. The study of these organisms is called microbiology, and scientists who study these organisms are microbiologists. Not all microbes cause disease; many are beneficial and even essential. Bacteria, in the digestive system, for example are important partners in digestion. Microbes that cause disease are sometimes informally referred to as “germs” or “bugs”.

The five main groups of pathogens are bacteria, viruses, protozoa, fungi, and helminths. Bacteria are simple, single-celled organisms that lack an organized nucleus or membrane enclosed organelles. They often have a cell wall (prokaryotes), and their cells usually are rod-shaped or spherical. Commonly known diseases caused by bacteria are diarrheal diseases, pneumonia, strep throat, tuberculosis, and anthrax. 

Viruses are particles of nucleic acid (DNA or RNA) surrounded by a protective coat that replicate within specific host cells and can spread from cell to cell. Infectious diseases caused by viruses include the flu, the common cold, AIDS, chickenpox, and hepatitis. 

Protozoa are single-celled, motile, eukaryotic organisms, found in the Kingdom Protista, that can be human parasites. A protozoan known as Plasmodium (over 170 species), causes malaria, an infectious disease that is one of the world’s top killers.

Fungi are made of eukaryotic cells (organized nucleus and membrane enclosed organelles). All fungi, with the exception of the yeast group, are multi-cellular organisms that absorb nutrients from the environment. Fungi can cause athlete’s foot, sinusitis, skin diseases, and vaginal infections.

Helminths (worms and flukes) are invertebrate animals, some of which are parasitic. Wuchereia bancrofti is transmitted to humans by way of the mosquito. The mature adults pass into lymphatic glands, obstructing lymphatic drainage and resulting in a disfiguring condition, known as elephantiasis.

Many microbes produce toxins or enzymes that damage host tissue. Bacteria produce two groups of toxins: endotoxins and exotoxins. Endotoxins are part of the cell wall of some types of bacteria and are released into host tissues when the bacteria divide or die. Exotoxins are secreted by bacteria into host tissues; they are sufficient to make a person ill even in the absence of the bacterial organisms.

There are seven different types of the exotoxin produced by the organism that causes botulism (Clostridium botulinum). Types A, B, E, and F cause a severe type of food poisoning, producing one of the most poisonous substances known to mankind. Types C, D, and E cause illness in mammals, birds, and fish. Botox, a pharmaceutical, is a purified and diluted A neurotoxin used for clinical and cosmetic purposes (e.g., to remove wrinkles by paralyzing forehead muscles for several months). Viruses must invade a cell directly in order to reproduce and in the process disrupt normal host cell functions. In many cases, the assembly and release of new virus particles causes the cell to lyse or rupture, releasing the new virus particles which can go on to infect more cells. Fungi and protozoa also can release toxins and enzymes that destroy host tissues. Some protozoa, such as the parasite that causes malaria, directly invade host cells.

Many signs and symptoms of disease are brought on by the host’s immune system in response to pathogen invasion. Fever, for example, is an attempt by your body to kill invading microbes that are sensitive to changes in temperature. Sneezing, coughing, vomiting, and diarrhea all are efforts by the body to rid itself of pathogens.

The most effective method of stemming the spread of infectious disease is through vaccination. Vaccines consist of weakened or killed microbes, or just components of a pathogen, and stimulate the body’s natural defenses—the immune system—to combat infections. Vaccination has eliminated smallpox, nearly eradicated poliovirus from much of the world, and drastically reduced the incidence of childhood infections, such as measles, mumps, and whooping cough, at least in the developed world. Influenza vaccines are available to reduce the occurrence of seasonal flu, although the shot must be given yearly due to the extreme variance of the influenza virus from season to season. Vaccines for other infectious diseases, especially HIV, still are being sought.

Antibiotics are effective for many types of bacterial infections (although they are entirely useless against viruses). But increasingly, bacteria are becoming resistant to the arsenal of antibiotics at our disposal. Very few drugs work well against viruses (anti-viral drugs for influenza and HIV were discussed in the previous two slides). Anti-fungal drugs exist, but their use is limited.

There are no vaccines against protozoan parasites, and other medications against them are becoming ineffective. Therefore, protection from insect vectors such as mosquitoes and control of mosquito populations are crucial strategies in containing the spread of insect-borne diseases, such as malaria. Good sanitation, water purification, hand washing, and proper cooking and storage of foods all help to reduce the prevalence of infectious disease. In cases of highly contagious, often fatal diseases, quarantine is employed as a means of preventing the spread of disease through a community. However, regardless of the disease, it is wise to limit contact with other individuals when ill.

As major decomposers, fungi serve an important role in ecosystems.  Without decomposition, there would not be enough available nutrients to sustain or create new life. 

Many fungi are parasites and thrive on living things, which is an association harmful to the host.  Fungi cause plant diseases such as black spot, corn smut, wheat rust, and mildews that affect a variety of fruits.  Most of us are familiar with fungi that cause human discomforts like athlete's foot, ringworm, and thrush.

Other kinds of fungi live together in a mutually beneficial relationship with other organisms.  Lichens are symbionts of a fungus and a green algae, or a cyanobacterium.  Mycorrhizae are mutualistic relationships between fungi and the roots of vascular plants.  Fungi cells supply the plant with more nutrients and water than normally would be absorbed by the roots alone, and the plant provides the fungus with products of photosynthesis.

Viruses are submicroscopic particles that can be seen only with a powerful electron microscope. They are not cells, but consist of genetic material, either DNA or RNA, that is enclosed in a protective layer of protein. Viruses are able to enter certain host cells and, in fact, must invade a host cell to reproduce. Once inside living cells, viruses hijack cellular apparatuses necessary to make copies of themselves. For this reason, they are considered intracellular parasites. Viruses use the instructions contained in their genomes to produce virus components, both nucleic acid and proteins, which are then assembled to produce new virus particles. The new virus particles then can go on to infect other cells and continue the cycle of virus reproduction.

There are more than 20 types of known sexually transmitted diseases. These illnesses pass from one person to another, primarily through sexual contact, and can be caused by bacterial, parasitic or viral infection. The spread of many of STD-causing microorganisms can be prevented by the avoidance of sexual activity, or reduced by the proper use of latex (or polyurethane) condoms for all sexual contact. Prompt screening for and treatment of common STDs, such as chlamydia, also reduces the risk of spreading or contracting a more serious infection, such as HIV.

Chlamydia, the most commonly reported bacterium-related STD in the US, is caused by the bacterium, Chlamydia trachomatis. An estimated 2.8 million Americans are infected with chlamydia each year, but many do not seek treatment because the symptoms often are mild or absent. The highest rates of infection are among adolescents between 15 and 19 years of age. Chlamydia can spread during any kind of sexual contact; the risk of infection can be reduced by correct use of latex (or polyurethane) condoms. Even though initial symptoms may be negligible, the infection is particularly dangerous to women, because it can damage a woman’s reproductive organs and cause serious problems, such as infertility, ectopic (outside the uterus) pregnancy and recurrent pelvic pain. Chlamydia can be treated and cured effectively with antibiotics.

Gonorrhea is an infection of the reproductive tract, mouth or anus caused by the bacterium, Neisseria gonorrhoeae. More than 700,000 persons in the US are infected with gonorrhea each year. Gonorrhea is spread through sexual contact. Several antibiotics can be used to cure this infection, but drug-resistant strains of gonorrhea are becoming more prevalent in many parts of the world. Correct use of latex (or polyurethane) condoms reduces the risk of contracting gonorrhea.

Herpes is caused by the Herpes simplex virus (HSV). Herpes infections around the mouth (cold sores) usually are caused by a strain of the virus called HSV type 1. Another variant of the virus, HSV 2 typically infects the genitals and anal area. HSV can be transmitted through any kind of sexual contact, including oral sex, and can spread even when sores are not present. Once a person is infected, the virus will remain in certain nerve cells of the body for life. There is no cure for herpes, but some antiviral medications can shorten or prevent outbreaks. Condom (latex or polyurethane) use reduces, but does not eliminate the risk of contracting HSV.

Human papillamoviruses (HPV) are common viruses that cause warts. About 30 types of HPV are transmitted sexually, and as many as 24 million Americans are infected. Some forms of HPV are linked specifically to the development of cervical cancers in women. Precancerous changes in the cervix can be detected by a Pap smear (a simple test performed in a doctor’s office). A new vaccine, Gardasil®, protects against four types of HPV that collectively cause 70% of HPV-related cervical cancers and 90% of genital warts. Correct use of latex (or polyurethane) condoms reduces, but does not eliminate, the risk of contracting or spreading HPV infection.

Trichomoniasis is caused by a single-celled parasite belonging to the protozoan group. It is the most common, curable STD in young, sexually active women. This disease also affects men, but the symptoms (irritation and yellowish discharge) are more common in women. Trichomoniasis usually can be cured with a prescription antibiotic. The use of latex condoms reduces, but does not eliminate, the risk of transmission.

Transcript from "Viruses," National Center for Macromolecular Imaging
Viruses have always lived alongside us. They've killed many millions of us. Despite their enormous power to cause disease, they're dependent on us. Invisible and deceitful, viruses are microscopic parasites. Without a host, they cannot reproduce. As scientists discover the chemical rules by which each virus plays, they can control how a virus affects us.

Additional Virus information:
Calici viruses have been identified in humans, other primates, cattle, mink, swine, dogs, chickens, cats, and others. Calici
causes a common gastroenteritis that is rarely diagnosed due to lack of adequate laboratory tests. Symptoms include acute gastrointestinal problems with more vomiting than diarrhea. (CDC. Foodborne illness. Retrieved 03/12/2007 from Garry, R. The big picture book of viruses. Tulane University School of Medicine. Retrieved 7-11-2007 from http://www.tulane.edu/~dmsander/Big_Virology/BVRNAcalici.html)

Transcript from "Viruses," National Center for Macromolecular Imaging
Vaccines are a second antiviral weapon. A vaccine introduced into the body gives it a clue about its attacker. Now the body can successfully fight the virus because it knows its chemical name. Smallpox no longer exists because of a vaccine. More than ten human vaccines have been created. Researchers are working to create vaccines for two of today's deadliest viruses, HIV and Rota. Rotavirus causes the dysentery that kills millions of children in underdeveloped countries. The relentless quality of these viral parasites is also an opportunity. Scientists are looking for ways to use viruses to deliver genetic fixes to cancer cells and to brain cells damaged by Alzheimer's.

Additional Virus information:
Rota Virus is the leading cause of gastroenteritis in children, adults, and elderly patients in nursing homes. The virus generally is contracted through contact with contaminated food or water. (U.S. National Library of Medicine. Viral gastroenteritis. NIH. Retrieved 03/12/2007 from http://www.nlm.nih.gov/medlineplus/ency/article/000252.htm)

Transcript from "Viruses," National Center for Macromolecular Imaging
Virologists have identified about 4,000 viruses. Viruses are different from bacteria, fungi, algae, plants and animals, but viruses are parasites to all of them. To copy themselves and grow, they must first infect and hijack a cell. It's their structure that makes it possible for them to steal the life from a cell and use it. Though mostly genetic material, viruses usually have other chemicals such as sugars, fats and proteins. Of the identified viruses, more than one thousand infect people.

Additional Virus information:
O’Nyong-Nyong, a member of Togaviridae family was initially isolated in Uganda in 1959. Mosquitoes are the carriers of this virus that leads to high fever, arthritis and rash. (McGill, P. E., & Njobvu, P. D. (2001) Rheumatology in Sub-Saharan Africa. Clinical Rheumatology, 20, 163-167)

Hepatitis A, B, and C:  Hepatitis refers to many types of diseases that cause liver inflammation.
Hepatitis A is a virus transmitted through contaminated food or water or direct contact with a person who already is infected with the virus and is the mildest form of viral hepatitis infections. (U.S. National Library of Medicine. Hepatitis A. NIH. Retrieved 03/12/2007 from http://www.nlm.nih.gov/medlineplus/ency/article/000278.htm).

Hepatitis B is a virus that is transmitted by contacting blood and other body fluids from a person already infected with the virus. (U.S. National Library of Medicine. Hepatitis B. NIH. Retrieved 03/12/2007 from http://www.nlm.nih.gov/medlineplus/ency/article/000279.htm).

Hepatitis C is a virus that also is transmitted through body fluids and is the one of the leading causes for the need for a liver transplant in the United States. (U.S. National Library of Medicine. Hepatitis C. NIH. Retrieved 03/12/2007 from http://www.nlm.nih.gov/medlineplus/ency/article/000284.htm).

Rabies is a curable viral disease found in mammals that is usually transmitted through the bite of an infected animal. Most deaths associated with Rabies are due to failure to seek prompt medical attention.(CDC. Rabies. Retrieved 03/19/2007 from http://www.cdc.gov/ncidod/dvrd/rabies/introduction/intro.htm)

Polio, also known as Poliomyelitis, is caused by Poliovirus. Polio affects the spinal cord and may lead to paralysis. Transmission occurs by person-to-person contact, often via a fecal-oral or oral-oral route. (CDC. Poliomyelitis. Retrieved 03/13/2007 from http://www.cdc.gov/nip/publications/pink/polio.pdf)

Human immunodeficiency virus (HIV) is transmitted by contact with bodily fluids from an infected person. HIV destroys specific cells that are important for normal function of the immune system, often, but not always, resulting in the development of Acquired Immunodeficiency Syndrome (AIDS). (CDC. HIV/AIDS. Retrieved 03/19/07 from http://www.cdc.gov/hiv/resources/)

Lassa Fever is an acute viral infection named after the town in Nigeria where the first cases were discovered in 1969. Although it can be spread from person-to-person, the primary host is the Mastomys rodent (multimammate rat). These rodents are often found in human homes, greatly increasing the chance of passing the virus from rodent to humans. (CDC. Lassa fever. Retrieved 03/13/2007 from http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/lassaf.htm)

Dengue fever is a mosquito-borne disease that can be caused by four different Dengue virus serotypes. The illness can result in a range of clinical syndromes from mild to severe and can be fatal. (CDC. Dengue fever. Retrieved 03/21/2007 from http://www.cdc.gov/ncidod/dvbid/dengue/facts.htm)

Marburg causes a rare, but severe, hemorrhagic fever in humans and primates. This virus is in the same family as Ebola virus. The first outbreak in 1967 resulted from exposure to African Green Monkeys and their tissue. The monkeys had been imported to Marburg, Germany, for research purposes and for use in preparing polio vaccine. (CDC. Questions and answers about Marburg hemorrhagic fever. Retrieved 03/13/2007 from http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/marburg/qa.htm)

Yellow Fever is caused by a virus that is transmitted to humans through the bite of a female mosquito of several species. The disease only occurs in Africa and South America. Travelers to these regions are required to be vaccinated. (CDC. Yellow fever: Disease and vaccine. Retrieved 03/21/2007 from http://www.cdc.gov/ncidod/dvbid/yellowfever/index.htm, and http://www.who.int/mediacentre/factsheets/fs100/en/)
 
Rubella or German Measles is a viral disease that causes acute fever and rash in the infected individual. Rubella can be transmitted by a cough or sneeze of an infected person. The vaccine for the virus is included in the MMR immunization. (CDC. Rubella: In short. Retrieved 03/19/2007 from http://www.cdc.gov/nip/diseases/rubella/vac-chart.htm)

Mumps is an acute viral disease resulting in fever, muscle ache, and swelling of salivary glands. It is transmitted by direct contact with saliva or respiratory secretions of an infected individual. The vaccine for the Mumps virus is included in the MMR immunization. (CDC. Mumps. Retrieved 03/13/2007 from http://www.cdc.gov/nip/diseases/mumps/vac-chart.htm)

Measles is a highly contagious viral disease characterized by a red, blotchy rash. The vaccine for the Measles virus is included in the MMR immunization. While no longer common in the United States, travelers need to be aware that measles is a common disease in many countries. (CDC. (2005). Measles. Traveler’s Health: Yellow Book 2005-2006. Retrieved 03/13/2007 from http://www2.ncid.cdc.gov/travel/yb/utils/ybGet.asp?section=dis&obj=measles.htm)

Hantaan is a viral disease transmitted by rats that causes “hemorrhagic fever with renal syndrome” (HFRS). Common in Asia, particularly in Korea, more than 2000 cases were reported during the Korean war among the United Nations troops. Since urban rats are almost always infected with the virus, it is thought that ships harboring rats caused world wide dispersal of the virus. (Brooks, G. F., Butel, J. S., & Morse, S. A. (2004). Jawetz, Melnick, & Adelberg’s medical microbiology (23rd ed.). McGraw-Hill.)

Epstein Barr, EBV, or Epstein Barr Virus, is considered the most common human virus, infecting most people at some point in their lives. It is found worldwide. EBV is the cause of infectious mononucleosis in adolescents and young adults. Once infected, EBV will establish a dormant infection. Rarely, it can play a role in the emergence of Burkitt’s lymphoma and nasopharyngeal carcinoma. Transmission of EBV requires contact with the saliva of a person who is infected while the virus is in its active form (CDC. Epstein-Barr virus and infectious mononucleosis. Retrieved 03/19/2007 from http://www.cdc.gov/ncidod/diseases/ebv.htm)

Cowpox is a viral skin disease that is very rarely seen today in humans. In 1798, Edward Jenner used cowpox to create the first immunization to smallpox (a related disease) by scratching the fluid from cowpox sores into the skin of healthy people. Jenner noticed that people who had been infected with cowpox were not only immune to further infections by cowpox, but also, to smallpox. (Levin, N. A. (2007). Cowpox infection, Human. Retrieved 05/08/2007 from http://www.emedicine.com/derm/topic87.htm)

Influenza virus cause respiratory infections that range from mild to severe and can lead to death. Typically spread from human-to-human, influenza is combated each year through vaccines prepared to match viral strains predicted to be prevalent during the coming year. (CDC. Influenza (flu). Retrieved 03/13/2007 from http://www.cdc.gov/flu)

The Ebola virus causes Ebola hemorrhagic fever, an often fatal disease that affects humans and primates. Much is unknown about this virus, but it is believed to originate from an animal host that is native to the African continent. There is currently no treatment for infection. (CDC. Questions and answers about Ebola hemorrhagic fever. Retrieved 03/13/2007 from http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/ebola/qa.htm)
 
The Omsk virus is transmitted through an infected tick bite and causes neurological diseases and haemorrhagic fever. (Charrel, R. N., Attoui, H., Butenko, A. M., Clegg, J. C., Deubel, V., Frolova, T. V., Gould, E. A., Gritsun, T. S., Heinz, F. X., Labuda, M., Lashkevich, V. A., Loktev, V., Lundkvist, A., Lvov, D. V., Mandl, C. W., Niedrig, M., Papa, A., Petrov, V. S., Plyusnin, A., Randolph, S., Suss, J., Zlobin, V. I., & de Lamballerie, X. (2004) Tick-borne virus diseases of human interest in Europe. Clinical Microbiology Infection, 10(12), 1040-1055.)

The Junin virus (in the same family as the Lassa virus, arenavirus) causes Argentine hemorrhagic fever. Viruses in this family usually are associated with a specific rodent species. (Retrieved 03/19/2007 from http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/arena.htm)

A rabies-like virus, Duvenhage virus, is associated with bats in Africa and leads to rabies-like disease in humans. (Duvenhage virus. Retrieved 03/19/2007 from http://www.cdc.gov/ncidod/eid/vol12no12/06-0764.htm)

The Oropouche virus is an important arthropod-borne virus that infects humans and causes Oropouche fever. It is a health threat in subtropical areas of South and Center America (Retrieved 03/19/2007 from http://www.cdc.gov/ncidod/eid/vol11no10/05-0464.htm)

The Orf virus that infects humans is associated with sheep, goats and other small ruminants. Orf causes ulcerative lesions in infected individuals. (Retrieved 03/19/2007 from http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5503a1.htm)

The Coxsackievirus causes hand-foot-and-mouth disease, typically in young children. It is a mild disease that appears as a rash with very small blisters on the skin. (U.S. National Library of Medicine. Hand-foot-mouth disease. NIH. Retrieved 03/13/2007 from http://www.nlm.nih.gov/medlineplus/ency/article/000965.htm)

A rare viral fever, Chikungunya, is spread by mosquitoes to humans. Disease symptoms include fever, headache, nausea, and muscle and joint pain. (Retrieved 03/19/2007 from http://www.cdc.gov/ncidod/dvbid/Chikungunya/chikvfact.htm)

Bunyamwera viruses are associated with congenial defects in humans. (Retrieved 03/19/2007 from http://www.cdc.gov/ncidod/eid/vol1no4/calishr3.htm)

Herpes: Herpes simplex virus (HSV) has two froms: Type I, which generally causes fever blisters and cold sores in the mouth and on the lips; Type II, which is associated with the genital area, but can infect the mouth. (Retrieved 03/19/2007 from http://www.niaid.nih.gov/factsheets/stdherp.htm)