APA Dictionary of Psychology

a set of physiological and behavioral changes that are induced primarily by proinflammatory cytokines (e.g., interleukin, tumor necrosis factor) in response to infection, chronic disease, or injury. These changes manifest in such symptoms as feverishness, fatigue and listlessness, sleep disturbances, appetite loss, abnormal sensitivity to pain (hyperalgesia), diminished motor activity, anhedonia, and social withdrawal. Sickness behavior is considered both adaptive, because it facilitates recovery from inflammation, and motivational, because it prompts relief-seeking illness behavior in the unwell person. Also called cytokine-induced sickness behavior.