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How many different strains of a disease can coexist in a single population of hosts? What effect do different mechanisms of coexistence have on the properties of diseases? The principle of competitive exclusion (Armstrong and McGehee 1980; Levin 1970) states that no more species can coexist in a system than the number of resources or limiting factors allow, which can be thought of, somewhat imprecisely, as stating that a single trade-off can support only a single species – the one that deals best with that trade-off. Disease models describe a simple ecological interaction, with hosts acting as resources, to test the limits of competitive exclusion. Trade-offs for the disease often involve virulence, a trait of abiding interest to hosts.
In the absence of a trade-off between host mortality and transmission efficiency, the disease strain with the lowest virulence would always win out in competition, and diseases would be favored to evolve ever-reduced virulence. When such a trade-off between host mortality and transmission efficiency exists, the single strain that maximizes the basic reproduction ratio R0 will persist (see Boxes 2.2, 5.1, and 9.1; Bremermann and Thieme 1989). Ecological factors that affect this trade-off, such as host density, might favor higher or lower virulence (Ewald 1994a). However, in the absence of spatial or temporal variation in these factors, only one strain persists [but see Andreasen and Pugliese (1995) for a case in which coexistence is due to density-dependence in the host].
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