Whereas anorexia is a common behavioral response to infectious diseases, the reasons for and mechanisms behind this observation are still unknown. When it is considered on an evolutionary basis, the organism must have net benefits from anorexia. The first response to infection is the development of acute phase response (APR). The APR is triggered by microbial products and characterized by production of several cytokines known to induce anorexia. Several microbial products and cytokines reduce food intake after parenteral administration, suggesting a role of these substances in the anorexia during infection. Locally released cytokines may inhibit feeding by activating peripheral sensory fibers directly or indirectly, and without a concomitant increase in circulating cytokines. However, the final center for appetite or eating is the central nervous system (CNS). Thus, these peripheral signals must reach and interact with brain regions that control appetite. In addition, a direct action of cytokines and microbial products on the CNS is presumably involved in the anorexia during infection.