Test results have identified a Norwalk virus as the cause of the epidemic of acute nausea and vomiting in hundreds of students last month, University officials announced yesterday.

"It's almost a foregone conclusion that this was a food-borne virus," said Dr. David S. Rosenthal '59, director of University Health Services (UHS).

"It was a virus that caused the outbreak," said University spokesperson Joe Wrinn.

Officials said their "best hypothesis" is that the Norwalk virus was spread via the salad bar in the Freshman Union. The overwhelming majority of the 225 students who reported illness had eaten in the Union during the 72 hours before the epidemic.

Norwalk is a family of viruses that can cause outbreaks of food-borne illness, Rosenthal wrote in a December 20 letter that was mailed to the parents of all first-year students. The virus has an incubation period of 24 to 48 hours, Rosenthal wrote.

In an interview last night, the UHS director said that Norwalk is the most common cause of gastroenteritis in the country.

In fact, a Norwalk virus was the source of a recent epidemic on the U.S. Navy ship The Saratoga. That epidemic caused about 1000 people to fall ill.

Norwalk viruses are also responsible for 40 percent of non-bacterial diarrhea yearly, according to the Merck Manual, a medical reference guide to diseases.

Rosenthal said medical experts from the Centers for Disease Control and Prevention (CDC) in Atlanta had found a small, round virus in several students as well as a dining service worker.

Using DNA sequencing, experts from the CDC pinpointed the virus as a Norwalk.

"All the cases at Harvard sequenced the same," Rosenthal said. "It's the same virus in all these cases."

Officials said they are unsure whether the food arrived at Harvard contaminated or whether a student or dining hall worker passed the virus to the food during the course of the day.

Surveys pinpointed six foods as possible candidates for carrying the virus. Most of them were items in the salad bar.

Most of those who fell ill either ate in the Union in the hours before the epidemic or were in close contact with someone else who did, Rosenthal said.

The remaining few cases--a small fraction of the total number, Rosenthal said--are most likely the result of a low-level winter gastroenteritis.

'It's Over'