Among the vast varieties of microorganisms, relatively few cause disease in otherwise healthy individuals. Infectious disease results from the interplay between those few pathogens and the defenses of the hosts they infect. The appearance and severity of disease resulting from any pathogen, depends upon the ability of that pathogen to damage the host as well as the ability of the host to resist the pathogen. However a host's immune system can also cause damage to the host itself in an attempt to control the infection. Clinicians therefore classify infectious microorganisms or microbes according to the status of host defenses - either as primary pathogens or as opportunistic pathogens:

One way of proving that a given disease is "infectious", is to satisfy Koch's postulates (first proposed by Robert Koch), which demands that the infectious agent be identified only in patients and not in healthy controls, and that patients who contract the agent also develop the disease. These postulates were first used in the discovery that Mycobacteria species cause tuberculosis. Koch's postulates can not be applied ethically for many human diseases because they require experimental infection of a healthy individual with a pathogen produced as a pure culture. Often, even clearly infectious diseases do not meet the infectious criteria. For example, Treponema pallidum, the causative spirochete of syphilis, cannot be cultured in vitro - however the organism can be cultured in rabbit testes. It is less clear that a pure culture comes from an animal source serving as host than it is when derived from microbes derived from plate culture. Epidemiology is another important tool used to study disease in a population. For infectious diseases it helps to determine if a disease outbreak is sporadic (occasional occurrence), endemic (regular cases often occurring in a region), epidemic (an unusually high number of cases in a region), or pandemic (a global epidemic).

Answer this series of questions:

1. What is used to study a disease among certain groups?
2. What is it used for?
3. What is a way to show that a disease can be spread to others?
4. How many microorganisms cause disease?
5. What are the infectious ones grouped as?
6. Why can't Koch's proposal be morally applied?
7. What is beneficial to explore during a disease outbreak?
8. Who else can a host's immune system cause harm to>
9. What causes tuberculosis?
10. What is unable to be cultured in vitro?
1. Epidemiology
2. to study disease in a population
3. satisfy Koch's postulates
4. relatively few
5. as primary pathogens or as opportunistic pathogens
6. it would require experimental infection of a healthy individual
7. to determine if a disease outbreak is sporadic
8. to the host itself
9. Mycobacteria species
10. syphilis