“What disease causes bloody diarrhea?”

There are many possible causes. The details of your case narrow down the possibilities. Not having any more information, I cannot venture a guess. Common causes of an acute case might be infectious colitis, ischemic colitis, diverticular bleeding. For chronic bloody diarrhea, Crohns or ulcerative colitis are possible.

Polyps, cancer, vascular malformations, or hemorrhoids.