The source of the illness has not been conclusively identified more than a week after people began getting sick.

An outbreak of Legionnaires’ disease in New York City has killed three people and sickened more than five dozen since late July. This type of pneumonia is caused by the bacteria Legionella, which grows in warm water.

Adults who have been in the affected area since late July with flu-like symptoms, fever, cough, or difficulty breathing should seek immediate medical attention, health officials say.