Let's hope it's not AIDS and something more mild, like necrotizing fascitis.
An occlusion of the major mesenteric artery leading to the large and small bowel would be my choice. It's rarely diagnosed in time to do anything useful and is therefor almost always fatal...aside from being terribly, I mean awfully, truly hideously painful...the bowel tissue becomes gangrenous...
I can't believe I said that. Who knew I could be so cruel and vindictive???
HUHAHAHAHA!