Infections acquired during a hospital stay; also known as nosocomial infections. They may be recognized during or after hospitalization. They usually appear during hospitalization, but as many as 25% of infections related to surgery occur after discharge. Data suggest that in the United States 4.5% of all hospitalized patients can expect at least one such infection, which translates into almost 2 million infections annually. Thus this major public health problem accounts for 299 million patient-days in United States hospitals each year.