The organisms discussed so far are living members of the microbial world. In order to be alive, an organism must be composed of one or more cells. Viruses, viroids, and prions are not living and are termed agents. Many infectious agents consist of only a few of the molecules typically found in cells. ■ viroids, p. 366 ■ prions, p. 365
Viruses consist of a piece of nucleic acid surrounded by a protective protein coat. They come in a variety of shapes, conferred by the shape of the coat (figure 1.9). Viruses share with all organisms and agents the need to reproduce copies of themselves, otherwise they would not exist in nature. Viruses can only multiply inside living host cells, whose machinery and nutrients they must borrow for reproduction. Outside the hosts, they are inactive. Thus, viruses may be considered obligate intracellular parasites. All forms of life including members of the Bacteria, Archaea, and Eucarya can be infected by viruses. Although viruses frequently kill the cells in which they multiply, some viruses exist harmoniously within the host cell without causing obvious ill effects.
Viroids are simpler than viruses, consisting of a single, short piece of nucleic acid, specifically ribonucleic acid (RNA), without a protective coat. They are much smaller than viruses (figure 1.10), and, like viruses, they can reproduce only inside cells. Viroids cause a number of plant diseases, and some scientists speculate that they may cause diseases in humans.
Table 1.3 Comparison of Eukaryotic Members of the Microbial World
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