Prince George's County health officials sealed off a University of Maryland dormitory yesterday after a mysterious illness struck 100 teenagers over the weekend.

The high school students are staying on the College Park campus while attending a youth leadership conference. Those who were housed in LaPlata Hall were moved to other residence halls late Sunday, said university spokesman George Cathcart. He said no one will be allowed to stay in the quarantined dorm until every nonporous surface is disinfected.

The safety measure comes after 100 teenagers and five conference staff members were hospitalized Sunday with nausea, diarrhea or other symptoms, said Pat Sullivan, a spokeswoman for the county health department. Seven students remained hospitalized last night, their conditions listed as stable, Sullivan said.

At least 300 students, ages 14 to 17, are attending a two-week program on medicine and health care run by the Chicago-based National Student Leadership Conference.

Health officials have not identified what caused the students to become ill, but medical tests may reveal within 48 hours whether it was a bacterial infection, such as salmonella poisoning, Sullivan said.

She said it could take several weeks to determine if the cause was a viral infection such as the Norwalk virus, which recently caused flulike symptoms among cruise ship passengers.

Several students said yesterday that they believe their symptoms were caused by a box lunch the program provided on Saturday. It contained a turkey sandwich, potato chips, an apple, several Oreo cookies, a bottle of water and packets of mayonnaise and mustard, the students said. Cathcart said the lunch was prepared by University of Maryland employees.

The first students fell ill Saturday night. Jillian Levi, 15, said she was walking to a meeting about 7 p.m. when she felt nauseated. "I felt like I had a really bad stomachache," she said.

Dozens of other students also reported feeling queasy Sunday morning. Nadia Khan, 14, of Bowie said she was attending an 11 a.m. lecture on leadership when she felt her stomach "tumbling around inside."

"Twenty minutes into the lecture, everyone was outside throwing up," Kahn said.

She called her parents to pick her up. When they arrived, she said she felt better and decided to take the program's day-trip to Baltimore. "They said it was going to be one of the best trips, so I really wanted to go," she said.

Khan started vomiting again on the bus to Baltimore, as other program participants began to feel ill. Khan's stepfather, Geoff Trout, said he saw dozens of other ill students when he arrived at a Baltimore mall to pick up his daughter.

"You could see kids with their heads on the table," he said. "They clearly weren't feeling well."

Sunday night, the 100 ill students and five staff members were taken to four Washington area hospitals, according to Sullivan. Cathcart said the university relocated all students from LaPlata Hall later in the evening.

The county health department finished taking samples from LaPlata Hall late yesterday afternoon. Cathcart said the university would disinfect public areas of the dorm so that it will be safe for students to retrieve personal items.

Eirian DiSanto, a spokeswoman for the National Youth Leadership Conference, would not comment yesterday.

Khan, who hopes to work in the field of medicine, said the illness hasn't lessened her enthusiasm for the program. But she said the incident will likely change her dining habits.

"I probably don't want to eat what they give us now," she said.