A degenerative illness is one that slowly gets worse and, as things stand, cannot be turned around. A joint, a nerve, or the brain keeps losing what it can do, over months or years — not a cold that comes and goes. Wear-and-tear arthritis and some nerve and brain illnesses are talked about this way. The word is almost always medical; you would not call a bad habit degenerative in careful speech, though a newspaper might stretch it. Though actually, "cannot be stopped" is the tidy picture in the dictionary: some treatments can slow the slide, and researchers keep trying, but the name still tells you the usual direction — downhill, little by little.