As New York City grapples with its worst-ever outbreak of Legionnaires' disease , health experts are trying to understand why reported cases of the illness have risen across the country in recent years.There are 10 confirmed deaths and at least 101 people have been infected in the South Bronx in this most recent outbreak. In an effort to stop it, New York City Mayor Bill de Blasio has ordered any building with a cooling tower to flush and disinfect the tower within the next 14 days.But New York is not the only place to experience more cases of the disease. New York City Health Commissioner Dr. Mary Basset explained this week that more cases have been reported across the United States."In the United States, and in our city, the number of cases of Legionnaires' disease have been increasing since the year 2000," Basset said. "The reasons for that are not clear, but may have to do with better case ascertainment. We have better and better lab tests that enable us to make the diagnosis. And it also may have to do with the fact that we have an aging population, more people susceptible to the infection."Annual reported rates of Legionnaires' disease or, legionellosis, increased 217 percent to 3,522 cases in 2009 from 1,110 in 2000, according to a 2011 report from the U.S. Centers of Disease Control and Prevention. The report cautioned that actual rates were likely higher than those reported.On a larger pneumonia -based study, the U.S. Centers of Disease Control and Prevention estimates that between 8,000 and 18,000 people with Legionnaires' disease are hospitalized every year.Dr. Frank Esper, a pediatric infectious disease physician at University Hospitals Case Medical Center in Cleveland, said doctors are becoming increasingly aware that if they see a severe case of pneumonia, they should test for Legionnaires' disease, which might contribute to the increase in reported cases."We think about legionella when they get so sick they have to be at hospital," Esper said. "We call legionella an atypical pneumonia."But he said the number of cases may be far higher because many doctors do not do the test unless the patient is extremely ill."A lot of times we don't figure out what causes these infections," he said of patients who contract pneumonia.Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville, Tennessee, said epidemiologists and other disease detectives are also trying to determine whether there are other factors that are making people more susceptible to the legionella bacteria that causes Legionnaires' disease."Some of it might have to do with the fact that our population is older. If you're older on exposure, you're more likely to get sick," Schaffner said. "Added to that we have [an] increased number of people with underlying lung disease and, in particular, people who are immunocompromised."