The risks of hospital-acquired infection were, for many years, not seriously considered. However, current conditions increase risk, and the intense media and legal interest have led to the issue becoming a major medical concern. Hospital-acquired infections can be considered from four angles: the clinical dimension is the most obvious. It involves hospitalized patients, and to an increasing degree, hospital staff; the economic impact although difficult to assess, is a largely underestimated drain on hospital budgets; the legal problems have been recently highlighted by developments in jurisprudence; finally, there are ethical considerations associated with the behaviour sometimes found among health care workers, as a consequence of workplace risks. In this article we try to define and assess the significance of the major factors. We discuss the validity of various epidemiological and economic tools, and thus highlight the role of hospital-acquired infections in the working of hospitals.