Sickness behavior

Sickness behavior is the coordinated behavioral response to infection and inflammation, comprising fatigue, social withdrawal, anorexia, sleep changes, and cognitive slowing. It is not a maladaptive consequence of illness but an organized, evolutionarily conserved survival program orchestrated by the immune system through its action on the brain.

The phenomenon is mediated primarily by peripheral cytokines — interleukin-1β and tumor necrosis factor-α — that signal the brain to reallocate metabolic resources away from activity and toward immune defense. Sickness behavior demonstrates that the immune system does not merely defend the body; it commandeers the nervous system to enforce a behavioral quarantine. The study of sickness behavior is the study of how the immune system governs motivation.

This immune-driven behavioral control has profound implications for psychiatry. When cytokine signaling becomes chronic — as in autoimmune disease, chronic infection, or persistent stress — the same motivational circuits produce the symptoms of depression: anhedonia, fatigue, social withdrawal, and sleep disturbance. The cytokine theory of depression is not a metaphor. It is the recognition that clinical depression and sickness behavior share a common neuro-immune mechanism.