Waterloo Region’s first ever outbreak of hepatitis C has seen five people infected, public health officials say.

The outbreak was announced Tuesday afternoon, although it’s believed all five people were infected on Dec. 24, 2013.

All five, authorities say, were at Tri-City Colonoscopy Clinic on Sportsworld Crossing Road on that day.

Health officials say there is not believed to be any risk to any patients of the clinic on any other day.

Exactly what caused hepatitis C to be transmitted to five people remains under investigation.

“We may never know for sure exactly what happened that day,” Dr. Liana Nolan, the region’s chief medical officer of health, told reporters.

Authorities have determined that a different endoscope was used for each patient, as per protocol.

One possibility based on similar outbreaks elsewhere, Nolan said, is that medication vials were reused or flushing solutions were used for multiple patients.

“We may not have 100% certainty about what exactly happened, but this is the problem area that tends to occur,” she said.

The outbreak first came to light when one case of hepatitis C was reported to Region of Waterloo Public Health.

A thorough investigation left the health unit baffled.

“There were no risk factors. We weren’t really sure where this could have come from,” Nolan said.

Late in 2014, a second case was reported.

One link was found between the two cases – both patients had been at Tri-City Colonoscopy on Christmas Eve.

After that, officials began contacting others who had been at the clinic.

Last week, test results showed the hepatitis C virus in three of the other 10 patients.

Nolan said the clinic has been “very helpful” and co-operative through the process.

In a statement, clinic officials said they were “troubled” to learn of the outbreak and apologetic to anyone infected.

“We remain committed to upholding and improving our high standards for infection prevention and control and to ensuring that all patients continue to have access to high quality endoscopy services,” the statement read.

About 120 people are diagnosed with hepatitis C each year in Waterloo Region.

It is a viral illness commonly transmitted through blood, and can lead to significant liver issues.

Some hepatitis C patients live with the virus unknowingly for many years, only discovering it when liver damage becomes apparent.