A definition of infection as growth of a microorganism in an animal with any resulting host response will include essentially all of the infectious diseases of humans. Many of the body surfaces of humans that communicate with the external environment (e.g., the skin and the gastrointestinal and respiratory tracts) support a normal flora, but these microorganisms usually do not invade and cause disease. Under the right circumstances, however, elements of the flora can invade and produce an infection.A number of other terms are commonly used in describing the infectious diseases. Pathology...