[any microbiologists incorrectly assume that hosts and their pathogens are usually in a state of slow evo- lutionary change toward some optimal future state, usually of active cooperation. This is a grossly unrealistic idea. Both pathogens and hosts must normally maintain close-to- stable equilibria by making trade-offs between competing values, such as growth rates and defensive activities. At equilibrium, a unit of improvement of one adaptation would require more than one unit of loss of another. A leaner rabbit might run faster, but at some point the benefit of still greater speed would not be worth the added risk of starvation. Likewise, our fever response is presumably optimized, at least for historically normal conditions. Higher and more frequent fever would make us less vulnerable to pathogens but would be more than counterbalanced by the costs of tissue damage and nutrient depletion. This will be true as long as the environment stays constant. If circumstances change, some of the optima for both host and pathogen will likely change. If bacterial pathogens are artificially kept in check for many generations, this may select for a decreased fever response, but if our technology fails and we become vulnerable again, we might recover a heightened fever response.
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PAST VERSUS CURRENT EVOLUTIONM