A sudden and violent sickness with the primary symptom being total liquification and elimination of the entire intestinal tract over a period of days.
During ass flu the infected will experience defication with the consistency of burnt coffee every thirty minutes for at least three days. The accumulated stress will usually lead to an angy starfish with a peanut.
Ass flu is almost always contracted by eating at buffet restaurants frequented by mobs of geriatrics; Golden Coral, Fire Mountain Grill, or Luby's.
I didn't want to go Fire Mountain Grill because it looked like ass flu waiting to happen. When my friend Matt went there he got the worst case of ass flu ever. He said his starfish was destroyed by a peanut cluster.