A Legionnaires' disease cluster in the Harlem area of New York City has caused three deaths and over 80 people to be sickened. Here's what it is, symptoms and how it's spread.

As of Thursday evening, there had been three deaths, which was the same number as the day before. However, the city’s Health Department said there were now 81 confirmed cases, up from 73 the day before.

The cases are most likely linked to a cooling tower in central Harlem, health officials said. More than 60 people have fallen ill, including three people who have died, from Legionnaires’ disease in New York City over the past several weeks, according to the city’s health department.