An outbreak of Legionnaires’ disease in NYC has doubled in reported cases and deaths since last week. Officials have struggled to identify the source of the outbreak, but it has now been confirmed.

Officials at the NYC Health Department issued the latest update on the outbreak Monday. Fifty-eight people living in Central Harlem have been diagnosed with Legionnaires’ since July 25, while two people have died from it. On the positive side, officials have identified several cooling towers contaminated with the pneumonia-causing bacteria and have taken action to contain the spread. For now, however, residents in affected neighborhoods are still being advised to immediately seek medical attention if they develop flu-like symptoms.

The outbreak first caught the attention of health officials in late July. By last week, there were 22 reported cases and one death. All cases, including this latest batch, have occurred across five zip codes in Central Harlem: 10027, 10030, 10035, 10037, and 10039.

Legionnaires’ disease is caused by various Legionella bacteria, usually Legionella pneumophila. Naturally found in the soil and freshwater, the bacteria only truly become dangerous when they enter water systems that can aerosolize them into a fine mist. These systems include hot tubs, cooling towers, and humidifiers. Once aerosolized, people can easily breathe in these bacteria, allowing them to invade the lungs and infect certain immune cells called macrophages. The disease was named in reference to the first known outbreak of it, which occurred in 1976 at a convention of the American Legion in Pennsylvania.

Though treatable with antibiotics, Legionnaires’ is often deadly once symptoms appear. It has a general fatality rate of 10% even today and a fatality rate of 25% in health care facilities, according to the Centers for Disease Control and Prevention. But these infections are not spread from person to person, and outbreaks can be effectively stopped once the source of contamination is found.

The NYC Health Department also reported Monday that it had identified 11 cooling towers that initially tested positive for the bacteria. All affected towers have also completed the remediation (steps taken to clear the bacteria out) required by the health department, it added. While the current danger may be over, however, it can take up to two weeks for symptoms of Legionnaires’ to appear following exposure, and officials say they do expect more cases to be identified. So people living in the aforementioned zip codes should quickly see a doctor if they come down with a flu-like illness.

“Anyone in these zip codes with flu-like symptoms should contact a health care provider as soon as possible,” said Acting Health Commissioner Michelle Morse in a statement released by the department.

Legionnaires’ disease is estimated to cause around 8,000 to 10,000 hospitalizations a year in the U.S., but its incidence has been on the rise since 2000. Part of this increase is likely due to simply improved screening and testing, but external factors like poorly maintained buildings or climate change are probably playing a part as well.