Clostridium difficile can be cultured from the stool of 3 per cent of healthy adults but most people remain asymptomatic. Clinical disease develops when normal gut flora is disrupted, usually by antibiotic exposure, thereby creating conditions that favour C. difficile proliferation in the colon. Gastrointestinal diseases associated with C. difficile infection range from mild diarrhoea to fulminant colitis. Some 'silent' infections present with abdominal pain and distension but little or no diarrhoea. These features may indicate severe disease, which causes toxic megacolon.