This chapter talks about certain observations and conclusions arrived at through the study of ecological immunology. The rate of parasitism and the level of immune response, for example, are often varied among individuals and populations. This variation, however, can be explained by ecological or demographic factors. The chapter also looks at the various costs that immune defences incur, including the cost of evolving and maintaining a defence, and the cost of using defence. It looks at the limiting resources for immune defences – energy, food, and nutrients – and how the capacity to immune-defend affects host fitness, where organisms face the problem of how to allocate their limited resources to defend, versus other demands, in order to achieve a maximum possible fitness. Finally, the chapter looks at the various elements of defence, namely resistance and tolerance. Resistance is where hosts reduce parasite numbers, while tolerance is where the damage caused by the infection is limited.
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