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The deadly Legionnaires' outbreak in New York, explained

New York City Mayor Bill de Blasio at a press conference on Legionnaires' disease.
New York City Mayor Bill de Blasio at a press conference on Legionnaires' disease.
Andrew Burton/Getty Images

Since July 10, New Yorkers have been on alert following the discovery of an outbreak of Legionnaires' disease, a bacterial infection that spreads through water systems and can be deadly to smokers and the elderly.

Already, ten people have died, 100 have tested positive for the disease, and officials think more people will be diagnosed before the outbreak is contained. Here's what you need to know:

Exactly what happened in New York?

south bronx

People walking in the South Bronx, where the outbreak started. (Spencer Platt/Getty Images News)

There's an outbreak of Legionnaires' disease in the South Bronx, which is believed to have started with infected water-cooling towers — the kind buildings use as part of their air conditioning and ventilation systems.

Apparently, the bacteria that cause the disease (Legionella) made their way into five cooling towers and spread around to the unlucky people who live and work in the affected buildings.

The cooling towers have all since been disinfected, but the number of people who have become sick in the outbreak has already reached 100 as of Friday. Ten people have died. To prevent more illness, the city health commissioner ordered the owners of buildings with water-cooling towers to test and disinfect their units within two weeks.

According to the New York Times, this is the largest outbreak of the disease on record in the city, and health officials are urging New Yorkers with symptoms to seek medical attention immediately. They've also said the number of sick patients turning up in hospitals has slowed down, which suggests the outbreak may soon be over.

While the New York case is quite large, it's also pretty typical of Legionnaires'. As the state health department points out, "Most cases of Legionnaires’ disease can be traced to plumbing systems where conditions are favorable for Legionella growth, such as whirlpool spas, hot tubs, humidifiers, hot water tanks, cooling towers, and evaporative condensers of large air-conditioning systems."

What is Legionnaires’ disease?


The Legionella bacteria. (

It's essentially a more severe form of pneumonia, which is simply lung inflammation caused by bacteria. In this case, the bacteria are Legionella. (Legionellosis is the proper name for pneumonia caused by this bug, but most people just refer to it as Legionnaires’.)

Legionella bacteria thrive in warm water and can be found in hot tubs, cooling towers, hot water tanks, and large plumbing systems. According to the Centers for Disease Control and Prevention, these bacteria are "the most frequent causes of waterborne disease among humans in the United States."

Importantly, the disease does not spread from person to person. To get sick, people have to inhale bacteria-contaminated water droplets in their environment.

Two to 10 days after exposure to the bug, people will begin to feel a range of symptoms, including headaches, fever, chills, and muscle aches. After a couple more days, symptoms become more severe: cough, shortness of breath, chest pain, nausea, vomiting, and confusion.

There's also a milder form of Legionnaires' called Pontiac fever. It's caused by the same bacteria, but typically lasts for a couple of days — more like a typical flu.

Will Legionnaires' kill me?



If you're young and healthy, probably not. But just as pneumonia can be a very serious disease in the elderly or immunocompromised, so too can Legionnaires'. The disease can cause serious complications including lung failure or death in people with chronic lung disease or weakened immune systems. Smokers and people over the age of 50 are especially at risk, and according to the CDC, between 5 and 30 percent of Legionnaires' victims die.

But again, you'd have to come in contact with the bacteria to have any chance of getting sick, and (as I'm about to explain in the next section) Legionnaires' is quite rare.

How common is Legionnaires'?

The CDC estimates that between 8,000 and 18,000 people enter hospitals in the US with Legionnaires' every year. Compared with something like seasonal flu, which affects 20 percent of the population, it's uncommon. It's also far less common than pneumococcal pneumonia, which infects some 900,000 Americans each year.

How easy is it to catch Legionnaires'?

The disease doesn't spread from person to person. You need to inhale mist or vapor that's carrying the bacteria.

Is there a cure?

Yes, simple antibiotics will clear up the disease in most people. But again, if you're older, already sick, or a smoker, the disease can be more serious, leading to lung failure and possibly death.

Why does Legionnaires' have such a funny name?

The first outbreak of Legionnaires' was recorded in 1976 after a bunch of people at a convention of the American Legion got sick with the bacteria — hence the name.

How do you prevent Legionnaires'?

This is really a public health issue: You need to have safe and clean water systems that don't allow the bacteria to grow. It's up to the owners of everything from water towers to public fountains and hot tubs to make sure they're properly disinfected and have healthy pH levels.

This is why New York Mayor Bill de Blasio has proposed policy solutions in response to the Legionnaires' outbreak in the city. According to the New York Times, he's going to put forward legislation that would mandate all buildings register their cooling towers with the city and undergo routine inspections and that building owners immediately disinfect towers that have tested positive for the bacteria. This would make New York one of the first places to have such a rule.