Fever has been observed to have beneficial effects across a wide variety of organisms. One simple argument for its adaptive value is the fact that fever is very metabolically expensive, with an estimated 10% metabolic increase needed to raise the body temperature even one degree—it is unlikely that it would have persisted through as many phylogenetic levels as it has were it not useful for something (11). Even so, however, febrile responses have been observed in a wide variety of non-mammalian animal types. Many benefits of fever have been suggested, from increased immune response to direct combat of pathogens, even extending to the preservation of life long enough to ensure ability to reproduce (12). In a sense, fever could be considered comparable to inflammation on a whole-body level in that while there are clearly some instances in which it is a health hazard (such as autoimmune disease), there are also a significant number of benefits.
The pathogen-fighting qualities of increased temperature extend beyond effects on the behavior of the immune cells within the body, as well. Studies have shown that many viruses have survival rates in vitro up to 250 times higher at 37°C than at 40°C, temperatures that would not be unusual to achieve in the course of a febrile response (11). Although these experiments have not been performed in vivo, these results suggest that the increased temperatures would have harmful effects on the pathogen in addition to boosting the immune response of the host.
Figure adapted from Kluger 1979
Allowance of a febrile response in ectotherms has also been shown to increase survival after infection with live bacteria, shown above by the different survival curves corresponding to the temperatures at which the animals were kept. On the other hand, animals treated with antipyretics to prevent fever died within several days after exposure (13).