South Nassau Communities Hospital in Long Island, near New York City, announced on Wednesday that over 4,200 patients who had received insulin could have been exposed to hepatitis and HIV viruses. Currently, there aren’t any known cases reported, but the hospital urges its patients to be tested for hepatitis B, hepatitis C, and HIV.

“The risk of infection from this is extremely low,” the hospital said in a statement, according to Reuters. Patients might have been exposed to viruses from insulin derived from a pen reservoir, and not a single-use needle. This means it’s possible that several patients could have been infected, not just one. Typically, insulin pens are used to inject insulin into one patient, but blood can backflow into the pen, which is why they shouldn’t be used more than once.

Hospital spokesman Damian Becker told Newsday that though no one was actually seen reusing the insulin pen reservoir on more than one person, one nurse had reportedly said it was OK to do so. “Once that was said, we then followed through with a report to the state Department of Health,” Becker told Newsday.

According to Newsday, the hospital has sent out 4,247 letters to patients suggesting that they be tested for hepatitis B, C, and HIV. Though the chance of being infected by insulin pen misuse is very low, the hospital is offering hotlines for patients to schedule free and anonymous blood tests.

The Centers for Disease Control and Prevention has declared in the past that reusing insulin pens is dangerous and should never be done, yet despite these warnings, there have been plenty of stories of insulin pen misuse in hospitals around the U.S.

A similar situation occurred at the Buffalo Veterans Association Medical Center in New York, when 700 veterans may have been exposed to hepatitis C, hepatitis B, and HIV through insulin pen reuse. In 2008, Nassau University Medical Center in East Meadow sent out warnings to over 800 patients because a nurse may have reused an insulin pen on several patients.