Freeman said the existing knowledge about Norwalk is based largely on an "experiment" on some high school seniors.

"High school kids...went through the illness as a public service in the spring of their senior year," Freeman said. "They went through the illness several times."

No one knows, however, exactly how the agent triggers negative reactions in the body. But the results are clear: two to three days of illness, including stomach cramps, headaches, myalgias, low-grade fever, malaise, diarrhea and vomiting.

On rare occasions, according to the text, intravenous medicine must be used to combat severe diarrhea and vomiting.

When the epidemic began one night last month, doctors at the University Health Services said they had to hook up approximately three dozen students up to IVs. That fact suggests that Harvard students got a very large dose of Norwalk agent.

"The virulence of a case depends on the dose of the pathogen," Freeman said.

Epidemiologists from the U.S. Centers for Disease Control (CDC), however, have still not determined what food originally transmitted the virus. Five foods from the salad bar are suspected.

Students who got the primary dose spread the virus.

"People in the infirmary on the first night who were sick with the primary common source case gave the nurses on duty secondary cases," Freeman said. "Secondary cases could be more or less depending on the dose, but people tend to get less sick on successive occasions."

Experts said some students could still be getting sick with the same Norwalk agent that caused the December epidemic.

Luckily, a victim of the virus is unlikely to get sick again from the same virus for two to three years after the first attack, according to the text.

This resistance results from the build-up of antibodies. In order to find conclusive evidence that the Norwalk virus was responsible for the outbreak, epidemiologists from the CDC will test blood samples from victims of the epidemic for antibodies specific to the agent.

Only the "first wave of evidence makes the Norwalk agent seem most likely," Freeman said. The virus is very hard to identify because the symptoms are indistinguishable from many other gastroenteritis cases.

If the CDC tests do not find antibodies for Norwalk, the cause of the epidemic may remain a mystery.

Statistically, bacterial food poisoning is a much more likely cause than the Norwalk agent.

According to a statement from the CDC, about 92 percent of diagnosed food poisoning cases in 1987 were caused by bacteria. In the same year, 17 percent of food poisoning cases were caused by viral agents such as Norwalk