More than 40 students staying on the George Mason University campus are reported sick, and several have been transported to local hospitals, including one group of seven people.
The sick people are from a group of 80 students from all over the world, ages 15-22, who are at the school to attend the Congressional Awards Foundation Program.
The group was in the District Wednesday night when some students began having food poisoning-like or flu-like symptoms. An unknown number were taken to the George Washington University Hospital.
Around 1:40 a.m. Thursday, after returning to George Mason, more students developed similar symptoms. They called 911, and another seven students were transported to a nearby hospital.
Fairfax County health officials are still investigating what caused the illness, but they do not believe it was food poisoning. They believe it could be viral gastroenteritis, which causes vomiting and diarrhea.
"Originally they thought it might be related to the heat and dehydration, but they started to show more of a stomach-type virus or illness," said a member of Fairfax County Fire and EMS.
Health officials believe the virus was spread person to person by touching infected surface areas,News4's Shomari Stone reported.
The sick students are heading home. The program ended Thursday.
Health officials are working with the university to clean areas the sick students may have infected.
Stay with NBCWashington.com and NBC4 as the story develops.