Many individuals do not report minor disease symptoms; and these are called subclinical cases. These individuals encourage transmission of microbes and the maintenance of endemic levels of disease. Sometimes the microbes exist in a person for years or all their life; these people are called carriers and may transmit pathogenic microbes. Examples of microbes that may be “carried” are Salmonella, Mycobacterium, Streptococcus, and E.coli. Typhoid Mary carried the typhoid bacilli which had established in her gall bladder; and she spread the Salmonella through oral-fecal contact to many people. It is possible to be a passive carrier of microbes on our clothing; and the microbes never establish in you. The carriers of greater concern to health officials are the chronic and convalescent carriers because they provide a location for the microbes to grow and reproduce.

Microbes (diseases) reported are called clinical; and these cases are recorded and can be treated to remove the microbes from the population. If the microbes cause disease that is fatal or lethal, then mortality rates are recorded.

Severity of disease can also be described as chronic or acute. Chronic diseases linger for long periods of time and symptoms develop slowly, and chronic diseases may include TB, mononucleosis, and hepatitis. Acute diseases like cholera and yellow fever show development of severe symptoms that appear rapidly and often disappear rapidly too.