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 microscope, Bonomo found "minute living creatures" associated with the infection known as scabies. He described in detail how the animal reproduces, how it infects the skin, and how it is transmitted from person to person. Quite remarkably, Bonomo's research remained almost the only work on parasitic diseases for a century and a half.
Then after 1830, improved microscopes made possible a better understanding of such diseases, and the science of parasitology developed rapidly. The American biologist Joseph Leidy (1823-1891) discovered the presence of parasitic worms in hogs and cats in 1879 and recognized a parasitic amoeba at about the same time. The French chemist Charles Louis Alphonse Laveran (1845-1922) isolated the parasites that cause malaria and traced their life cycle in and out of their hosts. The French zoologist Félix Dujardin (1801-1860) conducted research on flatworms that is generally regarded as the forerunner of modern parasitology.
Credit as founder of this field of science is often given to the German zoologist Karl Georg Friedrich Rudolf Leuckart (1822-1898). Leuckart carried out extensive research on the role of insects, tapeworms, flukes, and other organisms in human disease. He published his discoveries in a monumental two-volume work between 1862 and 1876. Other researchers focused on the etiology of specific parasitic diseases. The German zoologist Fritz Schaudinn (1871-1906), for example, developed new staining techniques that revealed to him the role of parasites in syphilis, dysentery, and a variety of tropical diseases. In one of the most remarkable of such discoveries, the Scottish physician Sir Patrick Manson (1884-1922) showed in the early 1900s that elephantitis is caused by parasitic worms.
Since the turn of the century, dozens of parasitic infections caused by protozoans, flatworms, and invertebrates have been studied. Causative agents, methods of transmission, and cures have been found for the vast majority of these diseases. New parasitic infections keep turning up, however. For example, in 1996, Stanford pathologist Luis Fajardo and his colleagues reported on a lethal infection that they think may be due to a new and particularly nasty tapeworm.
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