ABSTRACT

Epidemiology is the study of the population dynamics of parasites. A parasite can be defined as an organism that lives in a close, obligatory association with a host of a different species for at least part of its life; the parasite depends on the host for one or more essential nutrients, and the fitness of the host is thereby decreased. Genetically based defense mechanisms are thus a likely consequence of parasitism. Some defenses are fixed or innate, others are facultative and dependent upon exposure to infection. The presence of facultative defenses gives rise to important differences between the population dynamics of parasites and free-living organisms.