A Legionnaires' disease cluster in the Harlem area of New York City has caused two deaths and 58 people to be sickened. Here's what it is, symptoms and how it's spread.

An outbreak of Legionnaires’ disease in New York City has killed two people and sickened 58 since late July. City health officials link the Central Harlem outbreak to cooling towers, structures containing water and a fan that are used to cool buildings.

Legionnaires' disease is caused by Legionella, a bacteria that can travel through water systems and devices. Although most healthy people don't contract it, it can be deadly for those who do.