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.

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.

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 it was prepared by university 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."

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.

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