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.