Within the context of this section of the book, the term parasite refers to organisms be-longing to one or two major taxonomic groups: protozoa and helminths. Protozoa are microscopic, single-celled eukaryotes superficially resembling yeasts in both size and simplicity. Helminths, in contrast, are macroscopic, multicellular worms possessing dif-ferentiated tissues and complex organ systems; they vary in length from a meter to less than a millimeter. The majority of both protozoa and helminths are free-living, play a sig-nificant role in the ecology of the planet, and seldom inconvenience the human race. The less common disease-producing species are typically obligate parasites, dependent on vertebrate hosts, arthropod hosts, or both for their survival. When their level of adaptation to a host is high, their presence typically produces little or no injury. Less complete adap-tation leads to a more serious disturbance of the host and, occasionally, to death of both host and parasite.
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