New York City Mayor Bill de Blasio speaks at a press conference to address the Legionnaire's disease outbreak in the city at Lincoln Hospital on Tuesday. Mary Bassett, commissioner of the New York City Department of Health and Mental Hygiene, stands at left. (Photo by Andrew Burton/Getty Images)

More than half of those who had been hospitalized have been treated and released, but many city residents remain on high alert about the outbreak. The death toll from the Legionnaires' outbreak in New York City has climbed to eight, officials reported Wednesday, and an additional 97 people have fallen ill from the disease. Here is a summary of the outbreak so far:

What's Legionnaires' disease, and how do you get it?

A kind of severe pneumonia, the disease is caused by a bacteria known as legionella,  named after an outbreak of respiratory disease among delegates of the American Legion attending a gathering in Philadelphia in 1976. It's not transmitted from person-to-person but by inhaling mist contaminated with the bacteria.

There are a number of outbreaks each year. The Centers for Disease Control and Prevention estimates that 8,000 to 18,000 people in the United States are hospitalized each year from the disease. Precise numbers are difficult because the symptoms of the disease look very similar to other types of pneumonia and can include fever, chills, muscle aches and cough. Plus, there's no way of knowing what the cause is unless a culture is sent to a lab for analysis, and that isn't always done.

Most people infected with Legionnaires' disease recover after being treated with antibiotics. Some individuals -- such as those who are over 50 years old, have chronic lung disease or weakened immune systems -- are at higher risk of becoming seriously ill when infected.

The situation in New York sounds scary. How is it different from other outbreaks?

The cluster of people who have gotten the disease is very large -- the largest in New York City's history -- and a higher proportion of them have died than is typical. Since July 10, health officials have confirmed at least 100 cases, and at least eight people have died. Health officials have been so concerned that they have urged New Yorkers with the symptoms to "seek medical attention right away."

Who are the people who have been infected?

Health officials are still investigating this, but they may have been residents or visitors of five buildings where the legionella bacteria has been found. According to a health department memo on Sunday, the cases confirmed at that point were in people ages 30 to 80 with a median age of 54, and 59 percent were male. At least 10 percent had HIV, and others had lung disease, diabetes and/or were alcohol or cigarette users.

Local news organizations have reported that Carmelo Quiles, 68, is one of those who died from the disease. Quiles, who had diabetes and emphysema, reportedly stopped at one of those buildings daily on his way to see his wife, who was living apart from him.

Officials said they found the legionella bacteria in cooling towers. What are those?

They're part of the rooftop machinery that controls hot and cold air in some buildings. In the wake of the outbreak, New York Mayor Bill de Blasio said he would seek legislation that would require those who own the buildings to register cooling towers and inspect them regularly.

Let me get this straight -- the bacteria live in water. What about other kinds of water? Also, it's  summer now. Should I be worried about swimming pools? 

Legionella typically lives in warm water naturally, and in the past it's been found in plumbing systems, decorative pools, fountains and hot tubs, according to the CDC.

The risk at recreational pools, which are kept at lower temperatures and are full of chlorine and other disinfectants, should be minimal. A 2001 study out of Italy that sampled swimming pools did find some bacteria in two out of 48 of them but noted that there wasn't a big risk of becoming infected.

New York City officials said the city's drinking water, pools fountains, water towers and buildings cooled with air conditioners are all safe.

Are things getting better or worse?

It's hard to say for sure, but health authorities have said the number of new cases is slowing down, and they believe the peak of the outbreak has passed. The five contaminated cooling towers have been disinfected, and officials have inspected a number of others in the area but have not found any more evidence of the bacteria.

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