A Legionnaires’ disease outbreak in Harlem has claimed a fourth life, with nearly 100 confirmed cases and more than a dozen people still hospitalized. For the first time, city officials have identified 12 cooling towers that tested positive for Legionella bacteria — including four city-owned buildings such as Harlem Hospital.

What are the symptoms of this type of pneumonia, called Legionnaires' disease, and how do you treat it? Here's what you need to know.