Published August 29, 2025

Legionnaires’ disease is a severe form of pneumonia caused by inhaling tiny droplets containing Legionella bacteria. The illness can be life-threatening, especially for older adults or people with weakened immune systems. Milder infections, such as Pontiac fever, can also occur, but the rise in serious cases of Legionnaires’ disease has health officials throughout North America concerned.

Outbreaks of Legionnaires’ disease are notoriously difficult to trace. Unlike foodborne illnesses, where a single event can often be identified, Legionnaires’ disease spreads through the air. The bacteria thrive in human-made systems (cooling towers, water tanks, plumbing), and when that water is aerosolized, or converted into particles light enough to be carried through the air, Legionella can travel for miles.

Joan Rose, director of the MSU Water Alliance and the Homer Nowlin Chair in Water Research at MSU, explained in an interview with CBC Listen, “warm weather, a nice warm biofilm, stagnant water…this is conducive to the regrowth of this bacteria.”

Rising risk in Michigan

The Centers for Disease Control and Prevention reports nearly a ninefold rise in cases of Legionnaires’ disease across the U.S. since 2000. Michigan has experienced a significant number of cases as well, including the 2014–2015 outbreak in Genesee County during the Flint water crisis, which resulted in at least 90 confirmed cases and 12 deaths.

Rose noted that Michigan’s aging buildings and water systems create conditions where Legionella can persist and grow.

“Older buildings will often have more stagnant water…and they may have iron buildup. Iron is a micronutrient for these bacteria,” she explained. 

Climate change and the rise of extreme climate events may further elevate the risk, with warming temperatures extending the window for bacteria to grow.

Protecting public health

Despite the complexity, prevention of Legionnaires’ disease remains possible. Routine surveillance, proper maintenance, and effective disinfection of cooling towers and plumbing systems are among the most important tools.

“Outbreaks may be difficult to trace, but they are not inevitable,” Rose said. “We need to focus on our distribution systems, our premise plumbing, our cooling towers.” She added that current regulations do not consistently mandate Legionella monitoring or the reporting of test results.

Because outbreaks can have devastating consequences, including death, Rose stressed that each case should be treated as an opportunity to learn and strengthen future prevention.

Listen: Joan Rose speaks with CBC Listen about Legionnaires’ disease

Story by Aja Witt