A third person with Legionnaires' disease died as investigators searched for the source of the outbreak that has sickened dozens, officials said Friday.

A spokesman for New York City Mayor Bill de Blasio said 11 more people had been hospitalized in the South Bronx since Thursday, bringing the total number of people infected in the outbreak to 57. De Blasio's spokesman said the third person who died was a middle-aged man with prior health conditions. He had been hospitalized for treatment at the time of his death.

On Friday, investigators found Legionella bacteria at the Opera House Hotel in the Bronx.

City health officials were working to calm concerns over the outbreak after two cooling towers tested positive for the bacteria. The water cooling towers are at Lincoln Hospital and Concourse Plaza on 161st Street, the shopping center located across the street from the Bronx County Hall of Justice. However, officials later said the tower at the hospital wasn't connected to the outbreak. 

Bronx residents should seek medical attention immediately if they experience any symptoms, which include fever, cough, chills and muscle aches. Legionnaires' disease is treated with antibiotics.

Dr. Edward Telzak, an infectious disease specialist with St. Barnabas Hospital, told News 12 that the Legionnaires' outbreak is something to be concerned about, but that it isn't rare.

There have been more than 2,400 cases of Legionnaires' disease nationwide so far this year, including more than 130 in New York City.