The variations in mechanisms used by different types of organism to fight infection are clearly interesting and important to parasitologists, medics and veterinarians. They are also important to ecologists working on particular systems, where an understanding of the overall biology is essential. But from the perspective of an ecological overview, the consequences for the hosts of these responses are more important, both at the whole organism and the population levels. First, these responses determine where individuals are on the spectrum from 'wholly susceptible' to 'wholly resistant' to infection - and if they become infected, where they are on the spectrum from being killed by infection to being asymptomatic. Second, in the case of vertebrates, the responses determine whether an individual still expresses a naive susceptibility or has acquired an immunity to infection.
These individual differences then determine, for a population, the structure of that population in terms of the numbers of individuals in the different classes. Many mathematical models of host-pathogen dynamics, for example, are referred to as S-I-R models, because they follow the changing numbers of susceptible, infectious and recovered (and immune) individuals in the population. The variations at the population level are then crucial in molding the features at the heart of ecology: the distributions and abundances of the organism concerned. We return to these questions of epidemic behavior in Section 12.4.2 and thereafter in this chapter.
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