HOW COMMON IS LEGIONNAIRES' DISEASE?
    Between 8,000 and 18,000 people are hospitalized in the U.S. each year with Legionnaires' disease, which is a type of pneumonia, according to the Centers for Disease Control and Prevention. The number of cases reported to the CDC each year has been rising, roughly doubling between 2000 and 2009.
    A study in New York City found 1,449 cases and 185 deaths between 2002 and 2011. That's an average of around 19 deaths per year.
    "Let's be clear that Legionnaires' disease has been a persistent health problem for years. A problem all over the country. A problem that has been slowing and steadily growing all over the country," Mayor Bill de Blasio said Tuesday.
    WHERE IT LURKS
    The bacteria can thrive in warm water and become especially dangerous when the water is turned into a mist that can be inhaled.
    Medical investigators have linked past outbreaks to public fountains, air conditioning systems, spas, showers and even the misters than keep fruit moist in supermarkets. In the case of New York City's outbreak, the infected people might have simply been walking by on the street when they inhaled the mist.
    Investigators are still trying to determine which, if any, of the five cooling towers are directly linked to the illnesses. The presence of the bacteria doesn't necessarily mean the equipment infected anyone.
    The disease is not transmitted person to person, nor can it be transmitted through drinking water.
    WHAT IS BEING DONE?
    The five cooling towers have all been decontaminated, and lawmakers are hurriedly crafting legislation they say could help prevent another such outbreak.
    Building owners would be required to register the location of cooling towers with the city. There will also be a schedule of mandatory inspections, plus rules mandating a prompt disinfection if bacteria are found.
    This type of regulation is rare in the U.S. but has existed for years in some other countries. Quebec, Canada, instituted similar rules after a Legionnaires' disease outbreak in Quebec City in 2012.
    HOW BAD IS THIS OUTBREAK?
    City officials suspect the outbreak is starting to ease. They believe it peaked on July 30, and they have seen a decline in new cases since then. The first contaminated cooling tower was discovered and cleaned on July 29.
    Health Commissioner Dr. Mary Bassett called it "the largest outbreak of Legionnaire's disease that we are aware of in New York City."
    It hasn't been nearly as bad as the episode that gave the illness its name — the 1976 outbreak that killed 34 people who had attended an American Legion convention in Philadelphia.
    Legionnaires' disease is treatable with standard antibiotics and has a fatality rate of between 5 and 10 percent.
    WHO IS AT RISK?
    Researchers say one reason for the increase in reported cases is that there are more elderly and chronically ill people than in decades past. Those people are more susceptible to the illness.
    City health officials said the victims of this outbreak all had other serious health problems.