What is a parasite

Any organism which lives, either temporarily or permanently, in, on, or, with another living organism from which it also obtains its nourishment is, broadly speaking, a parasite. The host is usually the larger and stronger of the two. This definition is a wide one and it could obviously include most microorganisms except those that are free-living. Yet, only eukaryotic microbes such as protozoa, helminths, and the arthropods, the latter of the animal kingdom, are defined as parasites. Fungi, which occupy their own kingdom, bacteria, which are prokaryotes, and viruses, which lack a complete cellular machinery, are obviously not covered by the definition. Nevertheless, some parasitic protozoa, such as Pneumocystis carinii, have only recently been found to be fungi (Cushion et al. 1990), and some protozoa, for example, the microspora, have been suggested to be, but still remain to be proven, to be fungi. The regrouping of these into the kingdom of fungi depends on studies of the sequences of small-sub-unit ribosomal RNA. Researchers have today compiled a rather robust map of the evolutionary relationship of microbes using these sequences. Three primary domains of organisms have been identified, including Archea (prokaryotes with a eukaryote-like transcriptional system), Bacteria, and Eukarya (eukaryotes). It is interesting to find the highly parasitic protozoa Giardia and Trichomonas to be the most primitive of all eukaryotes (Figure 2.1).

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