ABSTRACT
In the broadest definition of parasites, every infectious agent discussed up to this point, such as bacteria, fungi and viruses, can be included. However, traditionally the term has been reserved for an animal that derives its livelihood entirely from a larger animal or host. Our discussion here is limited to protozoa and worms and excludes anthropoids, which, except a few (e.g., body lice), are not strictly parasites but more often vectors for diseases (e.g., ticks). All parasites are pathogenic and the terms used to describe the diseases they produce end in the suffix
-iasis
(giardiasis) or
-
osis
(toxoplasmosis). Some parasites are shed in feces and produce infection in those who ingest them with food or water. Other parasites are present only during a portion of their life cycle in animals used for food and subsequently eaten by humans, whereupon they carry out sexual reproduction and produce eggs that get shed in feces. None of the parasites transmissible through the food or water is capable of multiplying outside the host.
