AAA is an Abdominal Aortic Aneyrusm - this is a weakness in the biggest artery in the body. 'Disecting' is a fancy way of saying it's got a big hole in in. So blood doesn't go to where it should go, instead it goes into the spaces around the organs rather than to the organs themselves. Untreated a disection like this will kill you. In surgery I *believe* the survival rate is around 10% (But I may be out of date on that)