Legionnaires' Disease Outbreaks Shine Light on Rising Number of Cases in US New outbreaks reported in a California prison and in Illinois.

 -- Outbreaks of Legionnaires' disease in New York, California and Illinois have drawn attention to a bacterial disease that experts say has been on the rise in recent years.

First discovered in 1976 at a convention in Philadelphia for American Legionnaires, reported cases of the disease have been steadily rising in recent years, according to federal health officials. The number of outbreaks this year is actually within the normal range, but the U.S. Centers for Disease Control and Prevention clarified that the number of cases per outbreak has been higher than normal and that cases are more common in late summer and fall.

The disease is caused by the Legionella bacteria and can cause potentially deadly case of pneumonia. The bacteria grow in warm water and can spread through air condition cooling units, fountains, hot tubs or large plumbing systems. The bacteria have to be inhaled to cause infection and cannot be spread from person to person.

Experts say it's unclear what could be behind the rising number of reported cases.

"The number of cases reported to CDC has been on the rise over the past decade," a spokeswoman for the CDC said in a statement to ABC News. "This may reflect a true increase in the frequency of disease (aging of the population, more high-risk individuals, climate), increased use of diagnostic testing, or more reliable reporting to CDC."

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 CDC report. The report cautioned that actual rates were likely higher than those reported.

In New York City, the worst ever outbreak of Legionnaires' disease in the city left 12 dead and at least 100 infected this summer. In recent days, at least 50 people have been diagnosed with the disease in two different outbreaks in an Illinois town with 8 reported dead. In a California prison, six inmates were confirmed to have contracted the disease, but 95 were being watched to see if they developed the disease.

Between 8,000 and 18,000 people with Legionnaires’ disease are hospitalized every year, according to the CDC.

Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical School, told ABC News it’s difficult to figure out if the reported cases show an actual increase in disease or just an increased awareness.

The trend of rising cases has being going on for years, Schaffner said, noting that coming up with possible explanations left "all of us experts scratching our heads."

Additionally, the disease has popped up more often in certain areas, but again experts are unclear if it’s a rise in disease or testing, Schaffner said.

“In the northern tier of states there’s more Legionnaires' disease than there is in the south or far West,” he explained. “That’s been a pattern for a long time. Could this be a difference in testing practices and doctors in those areas?”

The other possibility is that in northern states there are older buildings that are more vertical with buildings that have cooling units where the bacteria can grow, Schaffner said, noting that the medical community needs to know more about how often doctors test for the disease.

Doctors may start treatment for pneumonia before determining if it is caused by legionellosis and then not report the case, he said.

"The variation of testing practices is something we don’t know enough about," he said.

While awareness of the disease has increased, so has the population susceptible to being infected, Schaffner said, pointing out that with an aging population and new medications that suppress the immune system, there may be more people out there susceptible to infection.

"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 an earlier interview. "Added to that we have [an] increased number of people with underlying lung disease and, in particular, people who are immunocompromised."

The Associated Press contributed to this report.