Infectious and many non-infectious diseases are associated with immune activation and the production of cytokines in the periphery and in the brain. Studies on experimental animals indicate that cytokines within the brain induce a depression-like behavioral syndrome, characterized by anhedonia, diminished interest in the environment, reduced libido, anorexia, weight loss, hypersomnia, psychomotor retardation, fatigue, and diminished cognitive functioning. Similar symptoms accompany medical conditions that involve immune activation in humans. Moreover, depressed mood and other depressive symptoms are induced in humans following cytokine administration. Brain cytokines also affect the neurochemical systems that are implicated in the pathophysiology of depression. Based on these findings, it is suggested that immune activation is involved in the etiology and symptomatology of 'depression due to a general medical condition'.