Health inspectors have figured out what caused hundreds of students at a Philly-area college to fall terribly ill — an outbreak of norovirus.

More than 200 students, faculty and staff at Ursinus College were sickened by the virus, the Montgomery County Department of Health said Monday. The diagnosis was confirmed by tests on two students performed at Phoenixville Hospital. The virus is commonly known as a "stomach bug."

“This is the agent we have suspected since this outbreak began,” Montgomery County Commissioner Valerie Arkoosh, a physician and interim medical director of the health department, said in a statement. “While the illness has been truly unfortunate, this has been a model of cooperation between the various health agencies and Ursinus. We will continue to be vigilant and work closely with Ursinus to focus on hygiene measures to reduce transmission.”

Dining halls at Ursinus were closed last week as sickness spread through the campus population. Classes resumed Monday after being canceled last week on Thursday and Friday, along with weekend events.

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With the virus, which the health department referred to as "an organism," identified, proper treatment can begin and the virus' spread on campus is expected to be curtailed.

Ursinus "officials indicated that the college has followed all health department instructions, and the illness is now expected to run its course," the Health Department said in its press release.

Norovirus typically causes diarrhea, vomiting, and abdominal pain.

The disease can be transmitted from "an infected person, contaminated food or water, or by touching contaminated surfaces," according to the CDC. "The virus causes your stomach or intestines or both to get inflamed. This leads you to have stomach pain, nausea, and diarrhea and to throw up."

According to the CDC, norovirus is the most common cause of acute gastroenteritis in the country, causes 19 to 21 million illnesses, contributes to 56,000-71,000 hospitalizations and 570-800 deaths every year. It is also the most common cause of foodborne-disease outbreaks.