Legionnaires’ disease is a deadly pneumonia caused by inhaling Legionella pneumophila or other species of Legionella bacteria that lurk in the organic matter lining drinking water pipes. The disease has been on the rise in the U.S., with cases quadrupling over the last 10 years. Headline-grabbing outbreaks often occur from a contaminated ventilation or hot water system in a hospital, and that’s where monitoring and prevention efforts have been focused.

Yet the majority of Legionnaires’ disease cases in the U.S. arise in private homes with no common link other than their water supply, underscoring that drinking water distribution systems are the ultimate source of outbreaks. “Furthermore, lab-scale type studies have illustrated that corrosion in drinking water pipes can stimulate the growth of Legionella,” says Amy Pruden, an environmental microbiologist at Virginia Tech and an author of the study. Corrosive water dissolves the protective mineral lining in pipes and then leaches iron out of old iron pipes. Iron is a micronutrient that boosts Legionella reproduction. The metal also reacts with and inactivates chlorine disinfectant that otherwise would kill the bacteria.

“Our team recognized that the conditions in Flint—a corrosive new water source in an aging water system—were just right for Legionella,” Pruden says. So before the city switched back to Lake Huron water in October 2015, the scientists decided to sample Flint’s water for Legionella, iron, and free chlorine. At the time of sampling, Flint had already experienced outbreaks of Legionnaires’ disease in June 2014 and May 2015, but the researchers were unaware because public health agencies did not notify the public about the outbreak.