Health officials concluded yesterday that Norwalk virus -- a bug infamous for causing stomach upset on cruise ships -- was responsible for 35 sudden but minor illnesses last month among students and a staff member at Carnegie Mellon University.

While the virus has been identified, it's still not clear how it spread on campus, said Guillermo Cole, spokesman for the Allegheny County Health Department.

Norwalk virus and similar germs lumped into a category called "noroviruses" can spread by way of food, but the Health Department ruled out food as a source in the CMU outbreak. While the close proximity of cruise-ship passengers explains how the viruses can spread on the high seas, the CMU cases didn't report a common location, like a single dormitory.

"These people did not have much in common, not only in terms of their food, but also their environment -- we couldn't put significant numbers of them together at the same place at the same time," Cole said.

Noroviruses are found in an infected person's feces and vomit. The viruses can be ingested when a sick person fails to wash hands after using the bathroom and then handles food that someone else eats.

That same dirty-handed person can contaminate surfaces and, through close contact, other people.

Noroviruses are highly infectious and usually cause nausea, vomiting, diarrhea and some stomach cramping. Lasting one or two days, the symptoms are commonly called "stomach flu," although there is no relation to viral influenza.

The U.S. Centers for Disease Control and Prevention estimate that 23 million cases of acute gastroenteritis are due to norovirus infection.

Among the 232 outbreaks of norovirus illness reported to CDC from July 1997 to June 2000, the cause of transmission was not determined in 23 percent of the cases.