Clostridium difficileandEscherichia coliO157:H7 are increasingly recognized causes of bacterial toxin-mediated colitis in immunocompetent individuals. Infection with these organisms results in significant morbidity and sometimes fatal complications. In the immunocompromised patient population, life-threatening and disabling diarrhea results from gastrointestinal infection withCryptosporidium, cytomegalovirus, and numerous mycobacterial species.