Lupus is famously joked about as a recurring diagnosis in House.
The phrase reflects a running joke in which House's team repeatedly considers systemic lupus erythematosus while investigating mysterious symptoms. Lupus is an autoimmune disease in which the immune system can attack the body's own tissues, potentially affecting the skin, joints, kidneys, nervous system, blood, and other organs.
The joke became so associated with the series that “It’s never lupus” became a recognizable House catchphrase among viewers. The line does not mean lupus is rare or unimportant; it highlights how a plausible diagnosis can repeatedly appear in a differential without ultimately explaining the specific case.
House did eventually use lupus as a diagnosis in the series. That detail is often remembered because it reverses the expectation created by the running gag. The show also used other recurring diagnostic jokes, but lupus became the best-known example.