A parasite is a plant or animal that lives and feeds on another plant or animal. The existence of parasites was recognized as far back as 1500 b.c. The Ebes papyrus of that date mentions hookworm infections among the Egyptian royalty. Greek physicians also knew of parasites. Hippocrates, for example, wrote about intestinal disorders associated with flatworms. The association of parasites with so many kinds of infection led many physicians of the Middle Ages to conclude that parasites were actually created spontaneously as a result of disease. Thus, as scientists began to develop a better understanding of the role of parasites in causing disease in the eighteenth century, these discoveries also contributed to the downfall of the theory of spontaneous generation.

Probably the earliest argument for parasites as the cause of a disease appeared in a 1687 paper by the Italian biologist Giovan Cosimo Bonomo (1666-1696). Using a simple...