Air conditioners have been working overtime this hot summer, from those tiny window units to the massive AC towers that serve the tightly packed apartment buildings in major cities. And while they bring the relief of cool air, these contraptions also create the conditions for dangerous bacteria to multiply and spread.

One particularly nasty bacteria-borne illness is currently spreading in New York City using those enormous cooling units as its vector: Legionnaire’s disease. The bacterial pneumonia, which usually recurs each summer in the US’s largest city, has sickened more than 100 people and killed five in a growing outbreak.

If you don’t live in New York City or the Northeast, you may never have heard of Legionnaire’s, but this niche public health threat may not be niche for much longer.

Climate change is helping to make Legionnaire’s disease both more plentiful in the places where it already exists and creating the potential for it to move to new places where the population may not be accustomed to it. Cities in the Northeast and Midwest, where hotter weather meets older infrastructure, have reported more cases in recent years. Recently, Legionella bacteria was discovered in a nursing home’s water system in Dearborn, Michigan — one of the states, along with Ohio, Pennsylvania, Illinois, and Wisconsin, that have seen more activity in the past few years.

Anyone can contract Legionnaire’s disease by inhaling tiny drops containing the bacteria, and the symptoms — fever, headache, shortness of breath — appear within days. It can cause a severe lung infection, with a death rate of around 10 percent.

While healthier people often experience few symptoms, the more vulnerable — young children, the elderly, pregnant people, and those with compromised immune systems — face serious danger from the illness. Around 5,000 people die every year in the United States from Legionnaire’s disease,

© Vox