Doctors have identified a highly contagious virus as the likely cause of an outbreak that left more than 100 teenagers violently ill over the weekend.

Frederick Corder, the Prince George's County health officer, said last evening that although final results will not be available until the end of the week, several of the sick students tested positive for norovirus, a gastrointestinal viral infection. Frederick Corder, the Prince George's County health officer, said last evening that although final results will not be available until the end of the week, several of the sick students tested positive for norovirus, a gastrointestinal viral infection. Also known as the Norwalk-like virus, it has sickened hundreds of passengers on cruise ships in recent years.

The virus causes acute vomiting, diarrhea, cramps, nausea and a slight fever for 24 to 48 hours, and victims can sometimes remain contagious for up to two weeks. But officials said that transmission of the virus can be controlled by taking simple precautions.

The virus is transmitted through surface contact and cannot be treated by antibiotics.

The infected high school students, ages 14 to 17, were among a group of 300 at the University of Maryland's College Park campus for a two-week program on medicine and health care run by the Chicago-based National Student Leadership Conference. About 100 students and five conference staffers were hospitalized Sunday with nausea, diarrhea or other symptoms, health officials said.

Seven remained hospitalized yesterday afternoon, Corder said, and the others were given precise instructions on how to avoid spreading the virus.

"The key here is hand-washing," Corder said. He said infection can be prevented by washing hands after using the toilet; after having diarrhea or vomiting; after changing diapers or touching stool-soiled or vomit-soiled surfaces; and before handling food or drink.

The students' dormitory was sealed off by public health officials Monday, and occupants were moved to other residence halls. LaPlata Hall is quarantined, and no one will be allowed to stay there until it is thoroughly disinfected, university spokesman George Cathcart said.

"We will extend that cleanup effort to everywhere else these kids have been," Cathcart said.

Public health officials have not yet found the source of the virus. They are testing university employees, including those who prepared box lunches eaten by the students before the outbreak.

The norovirus is so contagious that one sick worker preparing the lunches could have infected all who ate them, health officials said.

Three more students felt ill yesterday, Corder said. There were no reports of students or staff members from outside the conference with norovirus symptoms, Cathcart said.

Some of the infected students who live nearby returned home over the weekend. Nadia Khan, 14, of Bowie spent Sunday night at home. Her stepfather, Geoff Trout, said no one else in the family has had norovirus symptoms. "We're doing fine," he said.

The norovirus infects about 23 million people a year, according to the Federal Centers for Disease Control and Prevention. Those who contract the illness need to drink plenty of fluids to remain hydrated, Corder said.

Eirian DiSanto, a spokeswoman for the Chicago-based leadership conference, said that the infected students are feeling better and have been leaving their dormitory to attend program events. "We are back to our normal schedule for the students," she said.