A fourth person has died from Legionnaires' disease in the South Bronx as investigators search for the source of the outbreak, CNN reported.

The New York City Health Department announced that 65 cases of the disease have been reported since July 10, up from 31 on Thursday. Twenty of those patients have been treated and discharged.

Earlier this week, Health Commissioner Dr. Mary Bassett said the disease cannot be spread from person to person, "but urged caution," The Los Angeles Times reported.

"We are concerned about this unusual increase in Legionnaires' disease cases in the South Bronx," Health Commissioner Dr. Mary Bassett said in a statement. "We are conducting a swift investigation to determine the source of the outbreak and prevent future cases. I urge anyone with symptoms to seek medical attention right away."

Legionnaires' disease is a respiratory bacterial infection that is caused by the bacteria Legionella. Most cases of Legionnaires' disease can be traced to plumbing systems where conditions are favorable for Legionella growth, such as hot tubs, humidifiers, hot water tanks, cooling towers, and evaporative condensers of large air-conditioning systems.

Groups at high risk for Legionnaire's disease include people who are middle-aged or older, people with chronic lung disease or weakened immune systems.

Bassett said legionella is usually prevalent in the summer and fall because "the mist from the cooling towers is more likely to be expelled during these hot months and the water temperature is more likely to be conducive to the growth of bacteria during hot months."

Five buildings in the South Bronx have tested positive for traces of the legionella bacteria, including The Opera House Hotel and Concourse Plaza. Four of the five sites have been remediated.

"As we continue to investigate the potential sources of this cluster, we remind New Yorkers that this is a very treatable disease and we urge anyone who might be experiencing symptoms of Legionnaires' disease to seek medical attention immediately," Bassett said.

New Yorkers with respiratory symptoms, such as fever, cough, chills and muscle aches, are advised to promptly seek medical attention. Symptoms of the disease also include headache, fatigue, loss of appetite, confusion and diarrhea.