Experts' attention is drawn on a bacterial disease that has been on the rise in recent years, with recent outbreaks of Legionnaires' disease in California, New York and Illinois.

Legionaries' disease has been first discovered at a convention in Philadelphia for American Legionaries, in 1976. According to federal health officials, in recent years the reported cases of the disease have been steadily rising. The U.S. Centers for Disease Control and Prevention (CDC) clarified that this year the number of cases per outbreak has been higher than normal. Cases were also reported earlier than usually since they would be more common in late summer and fall.

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

According to experts, it's still unclear what could be behind the recent rising number of Legionnaires' disease reported cases. A spokesman for the CDC said in a statement to ABC News that over the past decade the number of cases reported to the health agency has been on the rise. He explained that this may reflect increased use of diagnostic testing, a true increase in the frequency of disease due to more high-risk individuals, aging of the population, climate or even more reliable reporting to CDC.

Annual reported rates of legionellosis or Legionnaires' disease increased by 217 percent from 1,110 in 2000 to 3,522 cases in 2009, according to a 2011 CDC report. The report also warned that the actual rates were probably higher than those reported.

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

According to the CDC, between 8,000 and 18,000 people are hospitalized every year with Legionnaires' disease. An infectious disease expert at Vanderbilt University Medical School, Dr. William Schaffner, told ABC News that it's difficult to evaluate if the reported cases show an actual increase in cases of the infection cases with the Legionnaires' disease or just an increased awareness.

According to Schaffner, this trend of rising cases has being going on for years and the disease was reported more often in certain areas, but experts are still unclear if the explanation is related to new testing methods or just a rise in disease.