Norovirus is what caused more than 12 percent of Ursinus College's student body to get sick last week, officials said Monday.

Test results confirmed the highly contagious virus as the culprit behind the stomach illness that swept across the Collegeville campus, sickening at least 214 students, plus faculty and staff, since last Tuesday.

"This is the agent we have suspected since this outbreak began," said Montgomery County Commissioner Valerie Arkoosh, a physician and interim medical director of the county health department.

Commonly known as the stomach bug, norovirus is spread by person-to-person contact, according to the Centers for Disease Control and Prevention. It is also the leading cause of outbreaks from contaminated food, the CDC said, spread when a contaminated food worker touches ready-to-eat foods before serving them.

But how it emerged and spread on the Ursinus campus is unclear. And officials "will probably never know" the original source, said Montgomery County spokesman Frank Custer.

Campus dining facilities were temporarily closed last week for cleaning after a health inspection found 17 violations, though they were not necessarily related to the illness.

As of Sunday night, school officials said, 214 students had reported symptoms of vomiting, diarrhea, and abdominal pain. Ten of those became sick during the weekend, and some called in from home to report their symptoms, which typically subside after 12 to 24 hours.

"Very small numbers" of students continued to report symptoms Monday, Ursinus spokeswoman Esme Artz said.

Classes, which had been canceled late last week as the virus spread rapidly through campus, resumed Monday.