By Stephanie Stahl

PHILADELPHIA (CBS)We now know that the stomach illness outbreak at Ursinus College was caused by a norovirus. Lab test results came today as classes resumed at the Montgomery County school. Eyewitness News was told there was a significant decrease in the number of students at Ursinus reporting symptoms.

Trouble started on campus Tuesday last week and classes were canceled Thursday and Friday and weekend activities were limited. But now, things appear to be getting back to normal.

Ursinus college says “aggressive cleaning, sanitizing and disinfecting efforts” will continue through the week, after 214 students and staff reported symptoms of gastroenteritis. The Montgomery County health department now confirms it was a norovirus.

“It causes mini epidemics quite frequently,” said Dr. Edward Ramoska, an emergency physician at Hahnemann University Hospital. He says the virus has been well known for causing outbreaks on cruise ships.

“Is especially contagious in places where a lot of people are in close quarters, like a college campus,” he says. “It tends to occur a lot in the winter because people are clustered together. Most people will present with nausea, vomiting, stomach cramps, diarrhea.”

Norovirus is easily spread from person to person by touching something thats contaminated with the virus, which is invisible.

“When somebody goes to the bathroom, and they don’t wash their hands very well , they go touching the door knob or touching something else you an certainly spread it that way,” Ramoska said.

22 Ursinus students were treated at local hospitals. The virus causes intense symptoms quickly, for a short period of time. There’s generally  only one potential serious complication.

“People can get dehydrated, and so people can faint and things like that. That’s the more dangerous sides of the disease,” he says.