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

Four people in New York City's Harlem neighbourhood have died from an outbreak of Legionnaires' disease, an aggressive and potentially deadly form of pneumonia. The city has seen 99 confirmed cases with 17 people in hospital with the infection, according to the New York City Department of Health.