New York City is experiencing its second community cluster of Legionnaires’ disease this summer, with the latest occurring in the Bronx. According to the New York City Health Department , 17 people have tested positive and two have already died. Ten cooling towers in Bronx have initially tested positive for the bacteria, and an investigation is under way to determine which towers are currently causing infections in humans.

This comes after a larger Upper East Side outbreak earlier this summer that involved 94 cases and 11 deaths. Here’s what to know about the disease.

What Is Legionnaires’ Disease?

Legionnaires’ disease represents a serious form of pneumonia caused by a bacterial infection. This bacteria occurs naturally in freshwater but can become a health issue when multiplying in human-made water systems.

The bacteria usually grows in warm waters and spreads through cooling towers, hot tubs, showers, plumbing systems and occasionally decorative fountains.

How Do People Get Infected?

The most common way an individual can get infected with Legionella is by inhaling infected tiny water droplets that contain the bacteria. A common method for this to occur is when cooling towers release contaminated water droplets into the air. Less commonly, people can become infected if they aspirate contaminated water; if drinking such water and it enters the lungs instead of going down into the gut. Individuals with swallowing difficulties are more likely to get infected through this route.

Legionella disease is not generally contagious, meaning people do not spread the disease person-to-person. Thus, outbreaks that occur are usually a shared environmental exposure as opposed to infected people spreading bacteria among themselves.

What Are Common Symptoms?

Symptoms usually occur 2 to 14 days after being exposed, and it usually resembles the flu or common respiratory infections. These symptoms include fever, cough, shortness of breath, headache, fatigue and muscle aches. More serious cases can also result in confusion and mental status changes.

Although the majority of people who get exposed show no symptoms at all, for some individuals the disease can progress rapidly leading to severe pneumonia, or infection of the lung and ultimately respiratory failure. About 1 in 10 individuals with Legionella die from complications, and risk usually depends on one’s age, severity of symptoms and how quickly treatment began.

Those most at risk are individuals older than 50 years of age, current or former smokers as well as those with weakened immune systems. This can include individuals with chronic lung and heart disease, diabetes or individuals on certain medications such as steroids. The vast majority of healthy individuals exposed to Legionella do not develop the disease.

How Is Legionnaires’ Disease Diagnosed?

Since the disease resembles other respiratory infections such as the flu and common cold, doctors cannot reliably diagnose Legionnaires’ disease by symptoms or even a chest x-ray. Instead, testing requires combining a Legionella urinary antigen test along with a lower-respiratory specimen like sputum to culture for the specific bacterial organism. During active outbreaks, like the one occurring currently in New York City, cooling towers can also be tested to compare those samples with patient isolates to determine whether they are genetically related.

Because Legionnaires’ disease is caused by a bacteria, treatment typically involves a 5-10 day course with an antibiotic, usually Azithromycin or Levofloxacin. Most patients recover with that course, although hospitalization is common because the infection can cause severe pneumonia. Approximately 20-40% of patients with Legionella require admission into the ICU, according to research published in the European Journal of Internal Medicine .

When Should You See A Doctor?

Anyone who has developed the classic symptoms of fever, cough and shortness of breath after a potential exposure to Legionella should seek medical attention immediately. This is particularly important for individuals who have visited or were in the vicinity of the cooling towers that have been linked to the current cluster of cases in the Bronx as well as those who are at risk for severe disease.

Legionnaires’ disease is serious but treatable. The best way to prevent it is by identifying and controlling contaminated water sources before it continues to expose additional individuals.