This story originally appeared on Vox and is part of the Climate Desk collaboration.

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people die every year in the United States from Legionnaire’s disease, many of them living in low-income housing with outdated cooling equipment where the bacteria can more readily grow and spread.

Legionnaire’s disease is a microcosm of climate change’s impact on low-income communities. As warmer temperatures facilitate the spread of disease, the most socially vulnerable populations are going to pay the steepest price.

The Collision of Legionnaire’s Disease, Climate Change, and Economic Disparities

Legionnaire’s disease was first documented after an unusually aggressive pneumonia outbreak during an American Legion conference in Philadelphia in 1976. Soon, Centers for Disease Control and Prevention scientists confirmed the cause of the mysterious illness: a previously unknown bacteria that was accordingly named Legionella. Legionella, unfortunately, is everywhere—in streams, lakes, and water pipes across the country.

But usually, it occurs in such low concentrations and is so remote that it doesn’t pose a threat to humans. Usually.

Now, city health officials have found the bacteria in the large cooling tanks that serve massive apartment buildings across New York City, particularly in Harlem. Cooling tanks are ideal places for Legionnaire’s to grow and spread. They’re filled with stagnant, warm water that is more hospitable to bacterial growth. Like an evaporative cooler, the systems convert warm stagnant water into cool air for apartment dwellers. They can spray mists laden with the bacteria into the open air, dispersing it across the surrounding air, where it can enter a person’s lungs when they inhale. According to the Environmental Protection Agency, 80 percent of Legionnaire’s cases are linked to potable water systems.