Pathogens Are Phylogenetically Diverse
Many types of pathogens cause disease in humans. The most familiar are viruses and bacteria. Viruses cause diseases ranging from AIDS and smallpox to the common cold. They are essentially fragments of nucleic acid (DNA or RNA) instructions, wrapped in a protective shell of proteins and (in some cases) membrane (Figure 25-2A). They use the basic transcription and translation machinery of their host cells for their replication.
Of all the bacteria we encounter in our lives, only a small minority are dedicated pathogens. Much larger and more complex than viruses, bacteria are usually free-living cells, which perform most of their basic metabolic functions themselves, relying on the host primarily for nutrition (Figure 25-2B).
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Some other infectious agents are eucaryotic organisms. These range from single-celled fungi and protozoa (Figure 25-2C), through large complex metazoa such as parasitic worms. One of the most common infectious diseases on the planet, shared by about a billion people at present, is an infestation in the gut by Ascaris lumbricoides. This nematode closely resembles its cousin Caenorhabditis elegans, which is widely used as a model organism for genetic and developmental biological research (discussed in Chapter 21). C. elegans, however, is only about 1 mm in length, whereas Ascaris can reach 30 cm (Figure 25-2D).
Some rare neurodegenerative diseases, including mad cow disease, are caused by an unusual type of infectious particle called a prion, which is made only of protein. Although the prion contains no genome, it can nevertheless replicate and kill the host.
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Even within each class of pathogen, there is striking diversity. Viruses vary tremendously in their size, shape, and content (DNA versus RNA, enveloped or not, and so on), and the same is true for the other pathogens. The ability to cause disease (pathogenesis) is a lifestyle choice, not a legacy shared only among close relatives (Figure 25-3).
Each individual pathogen causes disease in a different way, which makes it challenging to understand the basic biology of infection. But, when considering the interactions of infectious agents with their hosts, some common themes of pathogenesis emerge. These common themes are the focus of this chapter. First, we introduce the basic features of each of the major types of pathogens that exploit features of host cell biology. Then, we examine in turn the mechanisms that pathogens use to control their hosts and the innate mechanisms that hosts use to control pathogens.
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