DENVER, Colorado (CNN) -- A former hospital employee may have exposed hundreds, or even thousands, of surgical patients to hepatitis C after taking their fentanyl injections and replacing them with used syringes filled with saline solution, authorities say.

A hospital worker has admitted to secretly injecting herself and using unclean syringes for patients.

Kristen Diane Parker, who worked at Rose Medical Center in Denver, has admitted to secretly injecting herself in a bathroom and using unclean syringes as replacements for patients, investigators say.

She had hepatitis C, which she believes she contracted through using heroin and sharing dirty needles while she lived in New Jersey in 2008, authorities say.

She was a surgical technician at Rose from October 2008 to April 2009.

Nine patients who had surgery there during that time have tested positive for hepatitis C. Investigators are looking into whether they contracted the virus from Parker.

According to an affidavit filed by an investigator with the Food and Drug Administration, Rose Medical Center knew Parker tested positive for hepatitis C. She was counseled on how to limit her exposure to patients.

Parker quit after she was found in an operating room where she was not allowed to be. She subsequently tested positive for fentanyl. Hospital officials then contacted the DEA.

Health Library MayoClinic.com: Hepatitis C

Parker is in federal custody facing three drug-related charges. If she is found to have done serious harm to a patient, she could face up to 20 years in prison. If a patient dies due to her actions, she could face life in prison.

In a statement to police, Parker said, "I can't take back what I did, but I will have to live with it for the rest of my life, and so does everyone else."

Her attorney could not be reached Friday.

Rose Medical Center is contacting 4,700 patients who had surgery at Rose during the time Parker was employed there. However, hospital officials do not believe that that many patients were exposed.

"We are taking a very conservative and cautious approach by contacting everyone who had surgery during this broad time period," a statement on the hospital's Web site states. "It is likely that most of the patients who receive letters will not have been exposed to hepatitis C."

Another 1,200 patients may have been infected between May 4, 2009, and July 1, 2009, when Parker worked at Audubon Ambulatory Surgical Center in Colorado Springs. Audubon is also contacting patients.

According to the Centers for Disease Control and Prevention, hepatitis C is a contagious liver disease that can lead to cirrhosis or liver cancer.

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