No lights were on yesterday afternoon inside LaPlata Hall at the University of Maryland. Maintenance workers had scrubbed down the dormitory rooms, hallways and stair railings with strong bleach. A sign taped to the front doors said the building was closed and gave no indication of when it would reopen.

The dorm had housed a summer conference of about 300 high school students, but they were forced to move out this week after an outbreak of the highly contagious norovirus sent about one-third of them to hospitals. All of the ill students, who complained of severe nausea, diarrhea and stomach cramps, were given the green light to go home yesterday by Prince George's County health officials.

As the visitors departed the College Park campus, university administrators turned their attention to making sure there would not be a repeat outbreak when the university's more than 35,000 students return for classes in two weeks.

"We've gone through the major part of it," said Carreno T. Elmer, the county's deputy health officer. "It seems to be that it's on the downside."

Some university students have begun trickling onto campus. The football team arrived this week for summer practice, and members of the marching band and dormitory resident assistants will move in next week. But several students and visitors on campus yesterday said they were unaware of the virus outbreak or unconcerned.

"It's not as if it's SARS," said Julie Krakauer, who was sitting with her daughter, 19-year-old Carolyn Crow, in the university's newly renovated food court.

Krakauer said she and her daughter had driven from their home in New Hampshire yesterday and had not heard of the outbreak before a reporter informed her.

Crow just shrugged at the news. At her summer restaurant job, she said, she is forced to wash her hands so often that they become dry. If she touches her hair, or clears off a table, or handles a piece of food, she has to reach for the soap and water. "If it's still a big issue [when school starts], you just take precautions," she said.

George Cathcart, a university spokesman, said administrators are working on a plan to inform incoming students about any risk they might face. One of the most important messages, he said, is letting students know that washing their hands is the easiest way to prevent infection.

University officials also closed the Clarice Smith Performing Arts Center after someone involved with a music camp experienced symptoms similar to those caused by the norovirus. In addition, Cathcart said, three university employees have had similar conditions over the past two days. They have been sent home, and it has not been determined whether they were infected.

Pat Sullivan, a spokesman for the county health department, said she is awaiting lab results that could help health officials pinpoint the source of the outbreak. Those results could come late today or Monday, she said.

But Cathcart said the mystery may not be solved completely. "It's such a common virus," he said. "It could've come in from anywhere."

According to the Centers for Disease Control and Prevention, noroviruses are responsible for about 23 million cases of stomach flu, or gastroenteritis, each year. The virus is relatively resistant to freezing and high temperatures, and only a small strand is needed to infect a person. In most cases, the virus is passed by food. However, the source of contamination is undetermined in about a quarter of the cases.

Cathcart said he is confident that the campus will be disinfected before college classes begin at the end of the month.

"We really believe that we'll have everything in pretty good shape by the time these groups start coming," he said.