During an 11-year run, the bacteria that causes Legionnaires' disease increased 249 percent across the country and put nearly half the people who contracted it in intensive care units, federal investigators have found.

As alarming as those findings seem, public health investigators at the Centers for Disease Control and Prevention additionally found that at least 1 in every 10 infected with Legionella pneumophila, the bacteria that cause Legionnaires' disease, dies of the disease, which is a form of pneumonia.

Investigators also found that blacks were more likely to be infected than whites, a disparity attributed to housing differences, with blacks living more often in older and poorly maintained multi-resident structures.

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The new federal data arrives on the heels of last summer's outbreak that killed 13 people in the Bronx, and the recent revelations that cooling systems in several Long Island school districts were colonized by the microbes.

State health officials earlier this month mandated routine testing of air conditioning and water systems at schools, hospitals, dialysis centers and other sensitive sites in hopes of thwarting future outbreaks.

Dr. Pascal Imperato, founding dean of the School of Public Health at SUNY Downstate Medical Center in Brooklyn, said while the new CDC data are eye-opening, they're consistent with decades of statistics.

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"We have known for quite some time that about 20 percent of the general population will have antibodies to this organism," said Imperato, who was not connected with the CDC research. He called the bacteria "ubiquitous."

Imperato, a former New York City health commissioner and longtime Manhasset resident, said the presence of antibodies in the blood suggests a significant swath of the population has been exposed, but most are probably unaware of it.

Legionella bacteria can cause infection when inhaled on droplets of water. Older people and those with impaired immunity are more likely than the young and robust to become infected. He said the bacteria thrive in air conditioning systems and cooling towers.

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Cooling towers are the evaporative condensers or fluid coolers that are part of a recirculated water system.

"Even though these structures may be cleaned very well, the organism can reappear very soon afterward. And it's not a period of year or two, it can be a matter of months," Imperato said. Legionella bacteria also have an affinity for hot tubs, hot water tanks and decorative fountains.

Dr. Gayle Langley, of the CDC, headed investigators who found that not only has incidence risen, the bacteria are more prevalent in some parts of the country.

New York, the researchers found, has the highest rates of infection nationwide. The 249 percent jump in disease prevalence occurred between 2000 and 2011, Langley said. Between 2011 and 2013, half the people included as part of the research required admission to hospital intensive care units. All told, there were 1,426 cases from Jan. 1, 2011, to Dec. 31, 2013.

Langley and colleagues examined 10 so-called catchment sites, to garner a representative sampling of the United States.

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New York State health officials estimate the bacteria infect 200 to 800 people a year.

Mary Ellen Laurain, spokeswoman for the Nassau County Department of Health, said there was an average of 37 cases for each of the three years from 2012 to 2014. Drilling deeper, the county has had 47 cases through Sept. 30, the last month for complete reports. For the same period last year -- Jan. 1 through Sept. 30 -- there were 59, she said.

The Suffolk County Department of Health did not respond to Newsday's request for statistics.