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Background The number of reported cases of Legionnaires’ disease, a severe pneumonia caused by the bacterium Legionella, is increasing in the United States. During 2000–2014, the rate of reported legionellosis cases increased from 0.42 to 1.62 per 100 000 persons; 4% of reported cases were outbreak‐associated. Legionella is transmitted through aerosolization of contaminated water. A new industry standard for prevention of Legionella growth and transmission in water systems in buildings was published in 2015. CDC investigated outbreaks of Legionnaires’ disease to identify gaps in building water system maintenance and guide prevention efforts. Methods Information from summaries of CDC Legionnaires’ disease outbreak investigations during 2000–2014 was systematically abstracted, and water system maintenance deficiencies from land‐based investigations were categorized as process failures, human errors, equipment failures, or unmanaged external changes. Results During 2000–2014, CDC participated in 38 field investigations of Legionnaires’ disease. Among 27 land‐based outbreaks, the median number of cases was 10 (range = 3–82) and median outbreak case fatality rate was 7% (range = 0–80%). Sufficient information to evaluate maintenance deficiencies was available for 23 (85%) investigations. Of these, all had at least one deficiency; 11 (48%) had deficiencies in ≥2 categories. Fifteen cases (65%) were linked to process failures, 12 (52%) to human errors, eight (35%) to equipment failures, and eight (35%) to unmanaged external changes. Conclusions and Implications for Public Health Practice Multiple common preventable maintenance deficiencies were identified in association with disease outbreaks, highlighting the importance of comprehensive water management programs for water systems in buildings. Properly implemented programs, as described in the new industry standard, could reduce Legionella growth and transmission, preventing Legionnaires’ disease outbreaks and reducing disease.

The combined epidemiological and environmental investigation indicated that sewage-contaminated ground water was the likely source of this large outbreak. Long-term changes to the island's water supply and sewage management infrastructure are needed.