Lupus is the diagnosis repeatedly associated with the phrase “It’s never lupus” in House M.D.
The line became a recurring joke because House's team frequently considers systemic lupus erythematosus when evaluating patients with complex, multi-system symptoms, but the diagnosis rarely turns out to be correct. Lupus is an autoimmune disease in which the immune system can attack the body's own tissues.
The joke does not mean lupus is rare or unimportant in real medicine. Systemic lupus erythematosus can affect the skin, joints, kidneys, nervous system, blood, and other organs, and its symptoms can vary widely. That variety makes it a plausible differential diagnosis for many fictional cases.
House M.D. uses the phrase as a shorthand for diagnostic misdirection. It is a memorable catchphrase, not a clinical rule, and real physicians diagnose lupus through history, examination, laboratory testing, and other evidence.