Toxin-producing Clostridium difficile is the commonest cause of nosocomial diarrhoea and, as such, poses a major problem in our hospitals. The main population susceptible to disease is the elderly, for reasons that remain unclear. By contrast, carriage rates in neonates are high, but disease is low. The organism also has a major clinical impact in the immunosuppressed host, patients undergoing surgery (especially gastrointestinal) and those with severe underlying disease and longer hospital stay. Other interventions with high-risk associations are enemas, nasogastric and gastrostomy tubes and anti-peristaltic drugs. Data on the associated costs of C. difficile diarrhoea are not freely available, but one estimate is that in an average-sized district general hospital, 100 cases of C. difficile infection can be expected each year with an extra annual cost of 400,000 pound sterling and 2100 lost bed days.