Intensity-dependent pathology is an important criterion for distinguishing among parasites, and parasite body size is somewhat correlated with this criterion. The protozoans, bacteria, and viruses have short generation times, rapid reproduction inside the host, a tendency to induce immunity in surviving hosts, and a short duration of infection. Most parasites and pathogens that fit these assumptions are relatively small. The population dynamics of such parasites are well described by 'intensity-independent' (SEIR) models. Because such parasites are small, these models have gained the nickname of microparasite models.
The population dynamics of most parasitic worms are not well described by SEIR models. Because pathology increases with the number of worms in an infection (or intensity), and worms typically aggregate among hosts, pathology varies considerably from host to host. To model such worms more appropriately requires accounting for parasite intensity and aggregation, and more specifically keeping track of the number of individual parasites in the parasite population, the number of hosts, and the number of parasite free-living stages. Intensity-dependent models are better able to accommodate the biology of many species of adult parasitic worms. Because helminths are much larger than are protozoans, bacteria, and viruses, intensity-dependent models are termed macroparasite models.
While the terms macroparasite and microparasite have utility for modeling purposes, they have been inappropriately used as a coarse taxonomy, presumably because the prefixes in the terms focus attention to the body size of the consumer. Protozoa and smaller microbes are 'microparasites' and helminths and arthropods are 'macroparasites' even though some small parasites may be better modeled as macroparasites and many large parasites may be better modeled as microparasites.
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