AIDS is a very serious disease. It develops when a virus called HIV has damaged the body’s defence system so badly that the body can no longer fight off infections a healthy person would usually beat. The letters stand for Acquired Immune Deficiency Syndrome: acquired means you get it; immune deficiency means the defence system is weak; syndrome means a set of medical problems that appear together. Older news and many textbooks say AIDS usually causes death. Untreated, that is still true. Though actually, with today’s medicine many people with HIV never develop AIDS, and people who do can live far longer than in the 1980s — so the tidy old sentence is tidier than the present. In English we say it as one word, like “aids”, not as four letters. HIV is the virus; AIDS is the late, very serious stage. They are not two names for the same thing.