The virus responsible for last month's vomiting epidemic on campus is a well-known mystery.

While no one is sure exactly how the Norwalk agent causes vomiting and diarrhea, epidemiologists have identified this virus family and are familiar with its handiwork.

Big trouble comes in a little package. The virus, which varies in size from 26 to 34 nanometers, has caused illnesses from Hawaii to Japan to England. It was named after Norwalk, Ohio, where an outbreak struck an elementary school in 1968.

About 10 percent of gastroenteritis cases--or illness involving vomiting and diarrhea--are caused by Norwalk agents, according to Dr. Jonathan Freeman, assistant professor in the School of Public Health's epidemiology department.

But the Harvard epidemic was rare in its size and virulence.

"It is not usually an epidemic agent," Freeman said.

The virus is typically transmitted orally or through fecal matter. According to the text The Principles and Practice of Infectious Disease, the air may be responsible for carrying the virus in some cases.

The same book notes that foodborne spreading is rare. But Harvard, in completing its investigation of the epidemic, said the Norwalk virus spread via the salad bar in the Freshman Union.

Damage in Intestines

Through contact ranging from a handshake to a kiss, the virus enters the intestines, where it seems to cause the damage.

Under the affect of Norwalk agent, "villi are blunted," says the text. Villi, normally finger-like protrudances which absorb food particles, become squashed like mushrooms.

Microvilli also becomes shortened and spaces between the cells on the intestinal wall widen.

Diarrhea, one of the symptoms caused by the Norwalk agent, "is associated with the transient malabsorption of D-xylose and fat," according to the text. After the damage of the Norwalk agent, it may take two weeks for the absorption levels in the body to return to normal.

"Diarrhea washes out the enzymes which are used for digestion," Freeman said. "So that, in general, diarrhea breeds diarrhea."

"In this epidemic, diarrhea was a minor feature," Freeman said. The absorption levels probably returned to normal quickly, he said.