Hospital Characteristics Associated With Colonization of Water Systems by<i>Legionella</i>and Risk of Nosocomial Legionnaires' Disease: A Cohort Study of 15 Hospitals

The increase in reporting of nosocomial legionnaires' disease was attributable to increased use of urinary antigen tests; prior cases may have gone unrecognized. Risk of legionnaires' disease in hospital patients was better predicted by the proportion of water-system sites testing positive for Legionella than by the measured concentration of Legionella bacteria. Use of monochloramine by municipalities for residual drinking water disinfection may help prevent legionnaires' disease.