An outbreak of Legionnaires' disease in New York killed its fourth victim in the Bronx on Saturday, according to city health officials.

The total number of cases has risen to 65 since July 10, with 55 of them hospitalized. Recent reports say 20 people have already been discharged from the hospital, Fox News reported.

City officials said in a press release that most of those infected were older people with underlying medical problems, according to Reuters.

The affected areas include High Bridge, Morrisania, Hunts Point and Mott Haven, NBC York reported.

The disease is believed to have been spreading through contaminated cooling towers. Legionnaires' disease is a form of pneumonia that spreads when people inhale contaminated water, particularly small water droplets coming from the cooling towers or evaporative condensers of large air conditioning systems.

The city health department conducted an inspection on 22 Bronx buildings - 17 with cooling towers - and found five of them positive for the Legionella bacteria: the historic Opera House Hotel, Concourse Plaza mall, a Verizon office building, Lincoln Medical and Mental Health Center and Streamline Plastics Company. The five buildings have been disinfected to kill the bacteria.

Opera House Hotel general manager Julio Vargas confirmed that the hotel's cooling tower has been disinfected, adding that no hotel guest has gone ill from the disease so far, Reuters reports.

The Legionnaires' disease outbreak has caused Bronx residents to feel a sense of anxiety. Some have resorted to drinking bottled water for fear that their tap water has been contaminated, even as city officials assured them that the tap water is safe, according to The New York Times.

"The water supply in the south Bronx remains entirely safe. We don't know the source of this outbreak, but in recent months we have seen outbreaks associated with cooling towers and that's why we're focusing on them," Health Commissioner Mary Bassett said at a press briefing on Thursday, adding that every cooling tower in the Bronx is being tested, NBC New York reports.

Health officials, who said the spike in cases in the present outbreak is "unusual," are trying to determine the origin of the outbreak by interviewing patients about where they lived, where they worked and what routes they usually took when they walked.

New York City Mayor Bill de Blasio assured residents that there is no reason to be alarmed, as Legionnaires' disease can be treated if it is diagnosed early.

"The exception can be with folks who are already unfortunately suffering from health challenges, particularly immune system challenges," de Blasio said, NBC New York reported. "But for the vast majority of New Yorkers, if they were even exposed, this can be addressed very well and very quickly so long as they seek medical treatment."