Pseuomembranous colitis is an infection of the colon often, but not always, caused by the bacterium Clostridium difficile. Still, the expression "C. diff colitis" is used almost interchangeably with the more proper term of pseudomembranous colitis. The illness is characterized by offensive-smelling diarrhea, fever, and abdominal pain. It can be severe, causing toxic megacolon, or even fatal.