As with other biological organisms, infectious agents have evolved through natural selection to fill a particular niche in the global ecosystem, but in this case the organism is a parasite (Begon et al., 1996) having the ''body'' of another organism as the environment in which to grow or reproduce and from which to disseminate in order to colonize other suitable environments. Here we are, of course, interested in infections of humans, but it is useful to bear in mind that infection is a widespread phenomenon across the biosphere, including bacte-riophages infecting bacteria, viral infections of plants, bacterial infections of insects and nematodes, and so forth.
Numerous strategies have evolved allowing infections to continue to reproduce and disseminate (Mims et al., 2001; Davies et al., 1999). Rapid reproduction facilitates dissemination before the host's immune system can suppress it; alternatively a very slow rate of reproduction may allow the infection to establish and persist ''below the radar'' of the immune system. A highly virulent infection may kill the host before it has had the opportunity to disseminate (although in some cases consumption of the dead host by another potential host may be a potential route for further dissemination, as in hydatid disease); in some cases an infection may be highly virulent only when infecting a species not its usual host (e.g., arenaviruses causing hemorrhagic fevers or meningitis in humans, but little apparent disease in rodent reservoirs). Alternatively the infection may evolve with reduced virulence, providing a longer time window within which to spread to a new host. The duration of immunity evoked by infection is a significant determinant of observed infection behavior; for example, if immunity from measles infection were of only short duration, it would no longer simply be classifiable as an infection of childhood, whereas the long duration of immunity that it does produce necessitates a continual supply of newly susceptible infants.
The size of the infectious organism is also a factor in its reproductive strategy; the fact that single-cell organisms invest fewer resources in development than multicellular ones means that rapid reproduction and dissemination are easier to accomplish, whereas for multicellular organisms a longer lifespan is desirable to allow an adequate return on investment in development, which in turn implies more resources being invested also in defenses against the host's immune response. Broadly speaking, human infectious agents (and those of other animals) can usefully be divided into microparasites and macroparasites (Anderson and May, 1991; Thomas and Weber, 2001), although the distinction between the two is pragmatic rather than absolute. Microparasites are those organisms which reproduce directly in the host with relatively short generation times. They do so in such large numbers that it is impracticable to make a count of the numbers of the organism; in general upon recovery from infection there tends to be a period of immunity against reinfection. Viruses and bacteria are typical microparasites. Macroparasites, on the other hand, are generally larger organisms, as the name implies, which do not in general complete their reproductive cycle in a single host; their life cycles can be quite complex, with perhaps another animal species forming an intermediate host, or a free-living stage in the environment before maturity is reached in the definitive host. Infections with macroparasites result in many fewer organisms than with microparasites, and numbers of individuals in each individual stage in the life cycle can often be measured to provide a quantitative indicator of intensity of infection, typically correlating with the severity of disease experienced (but see page 178). Macroparasite infections tend to be of relatively long duration—a significant proportion of the host life time— and a degree of immune tolerance is usually evoked; if infection should be eliminated, for example as a result of treatment, immunity against further infection tends to be short-lived, so that repeated infections tend to be the norm. For reasons of space, the remainder of this chapter will concentrate on modeling microparasitic infections, with the modeling of macroparasite infections (a topic in itself) only briefly being discussed.
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