The researchers evaluated samples of hospital system water at 20 U.S. hospitals from 2000 to 2002. When cases of Legionnaires’ disease were identified, patient urine and sputum samples from 12 of the hospitals were tested to determine classification of Legionella, which has at least 48 strains.

The study, published in the journal Infection Control and Hospital Epidemiology, found that 70 percent of hospital water systems tested positive for Legionella species.

"Only those hospitals that had high levels of Legionella bacteria in their water systems had patients who contracted Legionnaires’ disease," senior author Dr. Victor L. Yu of the University of Pittsburgh School of Medicine said in a statement. "Proactive monitoring of the hospital water supply alerted physicians to the hidden risk of Legionnaires’ disease for their patients."

The average fatality rate of Legionnaires’ disease is 28 percent and it is estimated to be responsible for up to 20,000 U.S. cases a year -- many of them hospital-acquired.