A deadly outbreak of Legionnaires’ disease is ripping through Central Harlem in New York City. Local health officials report that about two dozen residents have been sickened by the pneumonia-causing bacteria so far this month.

The NYC Health Department issued its latest update on the outbreak Wednesday, following its first report last week. Since July 25, there have been 22 cases of Legionnaires’ disease diagnosed throughout the area, along with one death. Residents in the affected neighborhoods are being told to get immediate medical care if they start to experience symptoms of the disease, which typically resemble the flu.

“Anyone with flu-like symptoms should contact a health care provider as soon as possible,” said Deputy Chief Medical Officer Toni Eyssallenne in a statement from the NYCDOH.

Legionnaires’ disease is a type of pneumonia (lung inflammation) caused by various Legionella bacteria, though usually Legionella pneumophila. These bacteria are abundantly found in the soil and freshwater but aren’t dangerous to people in their natural state. The trouble starts when these bacteria enter and grow inside certain water systems that can aerosolize them into a fine mist, such as hot tubs, cooling towers, or humidifiers. People then breathe in this tainted mist, allowing the bacteria to reach deep inside their lungs and infect certain immune cells called macrophages, causing Legionnaires’ disease. A much milder form of infection, called Pontiac fever, occurs when the bacteria only reach a person’s upper respiratory tract.

Though not everyone exposed to the bacteria will get sick, Legionnaires’ can be a serious, life-threatening illness once symptoms appear, particularly for people at higher risk like smokers. Prompt antibiotic treatment can be life-saving, but most people will require hospitalization regardless. Even with treatment, roughly 10% of those sickened by Legionnaires die from it (in health care facility outbreaks, the mortality rate can be as high as 25%).

The NYC Health Department initially reported five cases of Legionnaires’ in the Central Harlem area last week, noting at the time that other people were still awaiting their test results. The current cluster of cases are scattered across five zip codes (10027, 10030, 10035, 10037, and 10039). Officials remain unsure about the source of the outbreak, but they suspect a contaminated cooling tower is to blame, and they are testing all operable towers in the area. They note that building plumbing systems are not at fault here, so residents can still drink water, shower, cook, and use their personal air conditioner as normal without fear. Legionnaires’ is also not spread from person to person.

While the overall risk to the public is low, officials are still advising anyone in the area to visit a doctor if they feel a flu-like illness coming on.

“Legionnaires’ disease can be effectively treated if diagnosed early, but New Yorkers at higher risk, like adults aged 50 and older and those who smoke or have chronic lung conditions, should be especially mindful of their symptoms and seek care as soon as symptoms begin,” said Eyssallenne.