Cachexia is loss of weight, muscle wasting, fatigue, weakness and decrease of appetite in someone who is not actively trying to lose weight. It can be a sign of various underlying disorders; when a patient presents with cachexia, a doctor will generally consider the possibility of cancer, certain infectious diseases (e.g. tuberculosis) and some autoimmune disorders.

The exact mechanism in which these diseases cause cachexia is poorly understood, but there is probably a role for inflammatory cytokines such as tumor necrosis factor alpha (TNF-α), which is also nicknamed cachectin for this reason.