Lupus is a name doctors give to more than one illness, not to a single tidy rash. When people say it now, they almost always mean an autoimmune disease: the body's defence system gets confused and starts to attack the body itself. Skin and joints are often involved — pain, swelling, tiredness, a face rash that can look a bit like a butterfly — but it can also trouble the kidneys, the blood, and other parts. It comes and goes. Some stretches are quiet; some are hard. You do not catch it from a friend like a cold. It is more common in women, and doctors cannot simply switch it off; they try to calm it. The most talked-about kind is systemic lupus erythematosus, often shortened to SLE, though more skin-only forms exist too. Though actually, the same Latin name was also used in the past for a different, eating-away skin disease that turned out to be a form of tuberculosis. So lupus is a family of medical names with a shared picture, not one thing. The word is Latin for wolf. Old doctors used it because a bad skin form seemed to eat the flesh the way people imagined a wolf would. That is a picture, not a fact about wolves, and it can sound more frightening than a careful explanation.