There is an error, although not a fatal one, in Dr. Mackowiak's thought-provoking application of evolutionary principles to fever [1]. He argues that the adverse effects of overwhelming sepsis can be understood "if one accepts preservation of the species rather than survival of the individual as the essence of evolution." Preservation of the species is not the essence of evolution, and neither is survival of the individual. In a sense, the unit of evolution is the individual gene. This approach was used by W.D. Hamilton [2, 3] to explain the seemingly paradoxical evolution of altruism and is now an accepted aspect of modern evolutionary theory.

Hamilton's key insight was the observation that an individual's genes can be passed on to the next generation, not only in offspring of that individual but also in offspring of that individual's siblings and cousins. It is therefore possible for a gene that tends to greatly increase the reproductive success of relatives while slightly decreasing the reproductive success of the individual to increase in frequency in successive generations and eventually to become predominant in the population.

A similar argument can be made about the response to overwhelming sepsis. This response evolved during a time when extended families presumably lived in close proximity. The rapid death of an individual with overwhelming infection could abort an epidemic that could damage the individual's children, siblings, and cousins. Accordingly, genes associated with such a febrile response would increase in frequency. This evolutionary process acts at the level of the individual gene and not on the species as a whole.