A nosocomial (or health care-associated) infection is one that is acquired in a hospital or health care facility (i.e., the infection was not present or incubating at the time of admission). For most bacterial infections, an onset of symptoms more than 48 hours after admission is evidence for nosocomial acquisition. To determine whether some infections such as legionellosis are hospital acquired, one must consider the usual incubation periods and determine whether the patient was hospitalized during that time period. Because hospital stays are getting shorter and more patients are treated in the outpatient setting, many health care-associated infections are not recognized during hospitalization. Infection control programs must therefore devise strategies for effective outpatient surveillance in order to accurately monitor nosocomial infection rates .