Air conditioning brings welcome relief from the summer sweltering heat, but a recent outbreak of Legionnaires’ disease in New York City caused by poorly maintained AC units has left 90 people sick and so far, killed three. It’s the largest occurrence in the city in a decade.

According to USA Today, experts warn other areas of the country may be at risk if AC units are poorly maintained. Legionnaires’ disease is a severe form of pneumonia — a lung infection — caused by the Legionella bacteria. The illness gets its name from a 1976 outbreak at an American Legion convention in Philadelphia, where many attendees became ill.

The Legionella bacteria thrive in warm water environments, such as air conditioning systems, hot tubs, cooling towers, and large plumbing systems. People contract Legionnaires’ disease by inhaling tiny water droplets contaminated with the bacteria. It is not contagious and cannot be spread from person to person.

Symptoms include high fever and chills, cough, shortness of breath, fatigue and chest pain. If not treated promptly with antibiotics, Legionnaires’ disease can lead to severe complications, especially in older adults, smokers, or those with weakened immune systems.

According to the Centers for Disease Control and Prevention, there has been a ninefold increase in Legionnaire’s disease since 2000 likely caused by hotter, more humid temperatures.

“When you have really hot environments, infrastructure is not keeping up,” said Jean Grassman, an associate professor at the City University of New York Graduate School of Public Health. “People are realizing, whether you’re talking about floods or cooling systems, they’re not designed for this. The fact that there's failures is not terribly surprising.”

Air conditioning can also contribute to other illnesses, such as “sick building syndrome,” where people who work in AC-cooled offices suffer more allergies and other symptoms such as skin irritation and rashes. The accumulation of dust and moisture inside air conditioning systems can also create the right environment for infectious microbes like fungus and viruses to flourish, says The Conversation.

It’s essential to keep air conditioning systems clean and well-maintained to prevent the spread of infections.