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The agents of human infectious diseases belong to five major groups of organisms: bacteria, fungi, protozoa, helminths, and viruses. Bacteria belong to the Bacteria domain, whereas fungi (yeasts and molds), protozoa, and helminths (worms) are classified in the Eukarya domain. Bacteria, fungi, and protozoa are unicellular or relatively simple multicellular organisms. In contrast, helminths are complex multicellular organisms. Viruses are noncellular and therefore are quite distinct from the other organisms.


Many of the essential characteristics of these organisms are described in Table 1–1.

  1. Structure. Cells have a nucleus or nucleoid (see below), which contains DNA; this is surrounded by cytoplasm, where proteins are synthesized and energy is generated. Viruses have an inner core of genetic material (either DNA or RNA) but no cytoplasm, and so they depend on host cells to provide the machinery for protein synthesis and energy generation.

  2. Method of replication. Cells replicate either by binary fission or by mitosis, during which one parent cell divides to make two progeny cells while retaining its cellular structure. Bacteria replicate by binary fission, whereas eukaryotic cells replicate by mitosis. In contrast, viruses disassemble, produce many copies of their nucleic acid and protein, and then reassemble into multiple progeny viruses.

    Furthermore, viruses must replicate within host cells because, as mentioned previously, they lack protein-synthesizing and energy-generating systems. With the exception of rickettsiae and chlamydiae, which also require living host cells for growth, bacteria can replicate extracellularly.

  3. Nature of the nucleic acid. Cells contain both DNA and RNA, whereas viruses contain either DNA or RNA, but not both.

TABLE 1–1Comparison of Medically Important Organisms


Eukaryotes (fungi, protozoa, and helminths) can be distinguished from bacteria based on their structure and the complexity of their organization.


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