The New York City Health Department is investigating a Legionnaires' disease cluster in Central Harlem that has killed at least two people and sickened more than 50 others, the agency reported on Aug. 4.

But what is Legionnaires' disease, how is it spread, and how prevalent is it in California?

Here's what you should know about the disease that health officials say is becoming more common in California and across the nation.

What is Legionnaires' disease?

Legionnaires' disease is a severe form of pneumonia, a lung infection, according to the Mayo Clinic . It is caused by Legionella bacteria. Legionella bacteria are found outdoors in soil and water, but these places don't usually cause infections. People typically get Legionnaires' disease in large buildings, where the bacteria ca

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