Fairfax County health officials are investigating the cause of an illness that has sickened more than three dozen students staying at George Mason University for a summer camp program.

Glen Barbour, a spokesman for the county health department, said health officials were alerted Thursday after students woke up vomiting and with stomach pain. Of the 40 or so affected students, more than a dozen were treated at a nearby urgent care center, where they received treatment for dehydration and were monitored for a few hours before being released.

University spokesman Daniel Walsch, said county officials initially thought the illness was food-borne,Walsch said. The cause of the sickness is viral gastroenteritis, also known as the stomach flu, county health officials say.

“Like any virus, it can spread from person to person,” Walsch said. “That’s why the county stresses wash your hands and be as hygienic as possible.”

The individuals who got sick were between 15 and 22 and were part of the Congressional Award Foundation Program, which brings kids from across the country to the D.C. area to learn about the federal government, Walsch said.

The first reports of symptoms started late Wednesday afternoon, with more reports accumulating throughout the day and early Thursday morning, Walsch said.

“People didn’t take it that seriously at first,” Walsch said.

He said the rooms in Piedmont Hall are being thoroughly washed by university officials, and for several days nobody will move into them, although another camp is expected to move in later this week.

Walsch said today was the last day of the program, and most of the kids had left the D.C. area, although there were some who lived in the area.