Legionnaires' disease continues to spread in a New York City outbreak that has killed three more people, bringing the death toll to seven, according to city officials Monday who have called for tighter regulations of water-cooling towers, which are believed to be the source of the outbreak. The outbreak is said to be the largest in at least three years, reported the New York Times

All of the victims are reportedly older adults with other underlying medical issues. The airborne disease has been contracted by at least 81 people, 64 of which have been hospitalized, according to Mayor Bill de Blasio's office.

Five South Bronx cooling towers were linked to the outbreak, including ones at the Opera House Hotel and the Lincoln Medical and Mental Health Center, after testing positive for the bacteria that causes the disease, reported the New York Times. Legionnaires' disease can be spread when people inhale water mist from these contaminated towers, but cannot be caught from another person. 



If it is diagnosed early, Legionnaires' disease can be treated with antibiotics. Depending on the outbreak, fatality rates can reach around 30 percent, according to the Center for Disease Control and Prevention. 

Those who live in proximity to the cooling towers should look for symptoms such as coughing, fatigue and confusion. New Yorkers can protect themselves by being cautious of water vapor and hot tubs.

An estimated 8,000 to 18,000 people in the U.S. are hospitalized with Legionnaires’ disease each year, according to the CDC. People over the age of 50, smokers and those with chronic lung disease or weak immune systems are most at risk, according to the National Institute of Health. 

There is an incubation period of two to 10 days for the disease, so it was expected that the number of cases would increase. Dr. Jay Varma, deputy commissioner for disease control at the New York City Department of Health and Mental Hygiene, told the New York Times that the city will be proactively monitoring the cases.

“We expect the case count to rise over the next several days because it reflects what has happened in the past,” Varma said. “But we are also confident we have done the most intensive, immediate work to cut off any risk, so we anticipate the number of cases will first rise, then fall again.”