Legionnaires' Disease

Legionnaires' disease is a severe form of pneumonia caused by inhaling water droplets or aerosols contaminated with Legionella bacteria, which thrive in warm water environments such as cooling towers, hot tubs, large plumbing systems, and decorative fountains. Around 6,000 cases are reported in the United States each year, though the actual burden is believed to be significantly higher — with an estimated 8,000 to 18,000 people hospitalized annually, as the disease is frequently misidentified as other forms of pneumonia. Cases have increased roughly ninefold in the U.S. from 2000 to 2018, with a disproportionately higher burden among socioeconomically vulnerable populations. Contributing factors include climate change, an aging population, aging infrastructure, and improved clinical awareness — though no single driver fully explains the rise. The disease is not transmitted person to person. Those most at risk include adults over 50, smokers, and immunocompromised individuals. With a fatality rate of roughly 10%, prompt antibiotic treatment is critical, and prevention centers on water management programs in buildings and healthcare facilities.