Fever is a part of the acute phase response to infection and inflammation. We now understand that fever is a complex physiological response that is aimed at facilitating survival of the host. The fever is induced by endogenous inflammatory mediators, such as prostaglandins and pyrogenic cytokines, that are released by immune cells activated by exogenous pyrogens. Although the pathways (humoral and/or neuronal) responsible for transfer of the pyretic signals from the blood to the brain are still under discussion, it is generally accepted that they act on the level of the anterior hypothalamus to raise the thermoregulatory set-point. Results of studies of the adaptive value of fever demonstrate an association between a rise in body temperature and a decrease in mortality and morbidity during infection. These data along with data from evolutionary studies provide a strong support for the concept that fever is a beneficial during infection in endotherms and ectotherms, vertebrates as well as in invertebrates. There are also evidence showing that fever may be used as a therapeutic tool, especially in cancer therapy. Based on the data reviewed in this article, it can be concluded that fever has evolved as a host defense mechanism which was preserved within the animal kingdom through hundreds of millions of years of evolution.