Rainwater left untreated in cooling towers atop city-owned Harlem Hospital fueled the Big Apple’s deadliest Legionnaire’s disease outbreak in a decade, the Rev. Al Sharpton charged Tuesday.

A law firm filed a lawsuit Aug. 20 against a New York City hospital’s construction company, alleging the company failed to treat bacteria-infected water in its cooling towers, leading to a Legionnaires’ disease outbreak.

Here’s what to know about water-cooling towers amid a cluster of Legionnaires’ disease cases in Central Harlem.

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.