Story highlights About 1,300 open-heart surgery patients could have been exposed to a bacterial infection, WellSpan York Hospital says

It says eight patients contracted nontuberculous mycobacterium, or NTM, and four of them died

The hospital says the infection probably was a contributing factor in the deaths

(CNN) A Pennsylvania hospital said Monday that it's telling about 1,300 open-heart surgery patients they could have been exposed to a bacterial infection after identifying eight patients who contracted nontuberculous mycobacterium, or NTM. Four of those patients died.

The Centers for Disease Control and Prevention stopped short of directly linking the deaths to the infection, as all four for of the patients had underlying medical conditions. But the hospital acknowledged that the infection probably was a contributing factor.

NTM is a naturally occurring bacterium found in soil and water, including, sometimes, tap water. People become infected by inhaling the bacteria, according to the American Lung Association . Most people do not become ill, although those with weakened immune systems are at risk. This can include someone recovering from an invasive procedure. The result is a pulmonary illness that can take more than a year of antibiotic treatment to cure.

WellSpan York Hospital in York, Pennsylvania, is contacting all patients who underwent open-heart surgery between October 1, 2011, and July 24, 2015.

"We know that the news of this potential risk of infection may be concerning to our open-heart patients, and we sincerely regret any distress that it may create for those patients and their families," said Keith Noll, senior vice president of WellSpan Health and president of WellSpan York Hospital.

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