A Legionnaires' disease cluster in the Harlem area of New York City has caused four deaths and sickened nearly 100 people. Here's what it is, symptoms and how it's spread.

There have now been 90 confirmed cases of Legionnaires' disease in central Harlem, and 15 people are currently hospitalized battling the disease, the city's health department announced.

New York City health officials are continuing to investigate a cluster of Legionnaires’ disease, a serious bacterial lung infection, where the number of cases has risen to 73, including three deaths.