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). QUESTIONS:

Q1:
1. What is the term given for the study of diseases in a population?
2. What is the study of diseases in a population called?
3. 
Q2:
1. What is Epidemiology used for?
2. What is the purposr of Epidemiology?
3. 
Q3:
1. What is a good was of showing that a disease is infectious?
2. How can one show whether a disease is infectious or not?
3. 
Q4:
1. What number of microorganisms are responsible for disease?
2. Disease is caused by what quantity of microorganisms?
3. 
Q5:
1. What terms are used when referring to infectious microorganisms?
2. What are infectious microorganisms grouped as?
3. 
Q6:
1. Why is Koch's proposal considered to be unethical?
2. Koch's proposal is perceived as being immoral for what reason?
3. 
Q7:
1. What is is useful to determine when studying an infectious disease's outbreak?
2. When an infectious disease outbreak occurs, what is it useful to explore?
3. 
Q8:
1. An immune system can cause harm to which organism on top of the one causing the disease?
2. Which organism can be harmed by a host's immune system on top of the one causing the disease?
3. 
Q9:
1. What is the name of the microorganism that is responsible for tuberculosis?
2. Tuberculosis is cause by which microorganism.
3. 
Q10:
1. The microorganism for which disease cannot be cultured in vitro?
2. The microorganism that causes which sexually transmitted disease cannot be cultured in vitro?
3. 
