Reverse Engineering the Febrile System.

Fever, the elevation of core body temperature by behavioral or physiological means, is one of the most salient aspects of human sickness, yet there is debate regarding its functional role. In this paper, we demonstrate that the febrile system is an evolved adaptation shaped by natural selection to coordinate the immune system to fight pathogens. First, we show that previous arguments in favor of fever being an adaptation are epistemologically inadequate, and we describe how an adaptationist strategy addresses this issue more effectively. Second, we argue that the mechanisms producing fever provide clear indications of adaptation. Third, we demonstrate that there are many beneficial immune system responses activated during fever and that these responses are not mere byproducts of heat on chemical reactions. Rather, we show that natural selection appears to have modified several immune system effects to be coordinated by fever. Fourth, we argue that there are some adaptations that coordinate the febrile system with other important fitness components, particularly growth and reproduction. Finally, we discuss evidence that the febrile system may also have evolved an antitumor function, providing suggestions for future research into this area. This research informs the debate on the functional value of fever and antipyretic use.