Public health officials are investigating two cases of Legionnaires’ disease possibly contracted at Rush Oak Park Hospital.

Officials with the Illinois Department of Public Health were at the hospital this week to test water samples for the bacteria, IDPH said in a statement.

It was unclear if the infected patients were exposed at the hospital.

The pair who contracted the bacteria were patients during only part of the time when they could have been exposed, IDPH said. One was a patient in May, the other in mid-July.

The health department said previous water samples taken at Rush Oak Park Hospital have tested positive for Legionella bacteria. IDPH said it was collecting additional information to investigate the bacteria’s origin.

Hospital spokeswoman Deb Song said the “likelihood of them contracting [Legionnaires’] here is unlikely.”

The hospital is working with IDPH to investigate the origin of the two Legionella cases, Song said.

“The source of these two cases are yet to be determined. The health and safety of our patients, visitors and staff is of the utmost importance,” Song said. “Rush Oak Park Hospital has a comprehensive water management program that follows the highest federal standards and CDC best practices.”

Legionella bacteria are transmitted through water droplets and can cause serious lung infections and possibly death. The bacteria mixes with the air in showers or fountains.

Incidents of Legionnaires’ disease have risen over 500% in the United States since 2001, according to the Centers for Disease Control. The disease was discovered in 1976 during an outbreak at a Philadelphia convention of the American Legion.