Cases of Legionnaires' disease have been increasing dramatically in the United States, with reported cases in August alone more than doubling from expected levels for that period, U.S. health officials say. 

Infectious disease experts say they cannot pinpoint a single reason for the surge, but believe a combination of factors is at play, including an aging population that is more vulnerable to illness, better diagnostics, increased awareness of the disease, and possibly environmental factors.

So far this summer, the Legionella bacterium has killed 12 people in New York's Bronx borough and eight at a veterans' home in Illinois. It is currently affecting scores of prisoners at the San Quentin State Prison in California.

The recent figures represent an acceleration from the most recent overall national data available, which show the number of cases reported to U.S. public health authorities more than tripled between 2001 and 2012.

"Whether that increase represents a true increase in disease or something else, we don't fully understand yet," Dr. Matthew Moore, a medical epidemiologist at the U.S. Centers for Disease Control and Prevention, said in a telephone interview.

According to the CDC's Sept. 4 weekly report on death and disease, there were 404 cases of Legionellosis - Legionnaires' and a related disease called Pontiac Fever - in the four weeks ended Aug. 29. That is more than twice as many cases as would be expected for the same four-week period in the past five years, said CDC spokesman David Daigle.

Legionnaires' was first reported at an American Legion convention in Philadelphiain 1976. In more extreme cases it can lead to severe pneumonia, respiratory failure, kidney failure and septic shock.

It is caused by inhaling mist or water droplets infected with Legionella, a naturally occurring waterborne bacterium. It cannot be spread from person to person. Legionella bacteria thrive in warm water, which may explain why infections are most common in the summer months or early fall, but they can happen at any time of the year.

According to the CDC, an estimated 8,000 to 18,000 people are hospitalized with Legionnaires' disease each year in the United States.

Moore said this summer's outbreaks have tended to be larger than the CDC has seen in the past. The New York outbreak was traced to a cooling tower outside of a building, which could have exposed anyone outside it to the bacteria.

Investigations are still under way into the cause of the outbreaks at the veterans' home in Quincy, Illinois, and at the San Quentin prison.

"As we complete those investigations, we'll have a better idea of why they happen and why so many people were affected," Moore said.

He did not discount the possibility of climate change as a possible contributor, noting that some research indicates increased rainfall could be associated with the increased cases of Legionnaires' disease. But Moore and several other experts cited steady increases in the age of the U.S. population as a particularly crucial factor.

"The older you are, the more at-risk you are to these infections. That is a large portion of this," said Dr. Matthew Sims, director of infectious disease research at Beaumont Hospital in Royal Oak, Michigan.

University of Pittsburgh infectious disease expert Dr. Amesh Adalja, speaking on behalf of the Infectious Disease Society of America, believes at least some of the increase in reported cases is related to greater awareness and the fact that doctors now routinely test pneumonia patients for the disease.

"There is a whole confluence of issues that may be resulting in more cases being reported," he said.