NYC "disease detectives" are investigating the source of a Legionnaire's outbreak in the South Bronx that has sickened 31 people and killed two others since July 10, Mayor Bill de Blasio and Health Commissioner Dr. Mary Bassett said Wednesday.

At a news conference, Bassett encouraged people with symptoms of Legionnaire's -- fever, coughing, breathing problems, chills and muscle pains -- to seek care immediately.

"This is a treatable disease if people with symptoms get medical care quickly," de Blasio said.

Health Department officials said that most of the cases have been reported from the High Bridge, Morrisania, Mott Haven and Hunts Point neighborhoods.

Bassett said "samples are already being taken" by health department workers to discern the source of the outbreak. "We have seen outbreaks in the past related to plumbing systems, cooling towers, evaporative water systems, spas (and) hot tubs," she said.

"I want to make clear that this is a common pneumonia --  one which, as the mayor said, is readily treated," Bassett added.

Legionnaires' disease is caused by a moisture-loving bacteria called Legionella which can be harbored in the evaporative condensers of air-conditioning systems (in greater use during sweltering summer days), hot water tanks, cooling towers, spas, hot tubs, humidifiers and whirlpool spas. Elderly people, smokers who are middle aged and people with weakened immune systems as a result of illness or taking immunosuppressive drugs are particularly at risk. Other symptoms of infection include headache, fatigue, loss of appetite, confusion and diarrhea.

In January, at least eight residents of Co-op City in the Baychester area of the Bronx were sickened by the pneumonia-like illness. Health officials determined then that a tower used to cool the heating and electrical systems of the complex was found to harbor Legionnaire's bacteria.