FAIRFAX, Va. (WUSA/AP) -- A likely outbreak of viral gastroenteritis caused about 40 students staying at George Mason University to become sick this week.

The Fairfax County Health Department said Thursday that it was investigating an outbreak that occurred among students participating in a program sponsored by the Congressional Award Foundation.

About 40 of the 80 students became ill. Physician Peter Troell (TROLL) with the county health department said the students likely contracted viral gastroenteritis, which causes vomiting and diarrhea. The disease is sometimes referred to as "stomach flu" even though it is not caused by influenza viruses. Germs like norovirus and rotavirus can sometimes be responsible for outbreaks of gastroenteritis.

Troell said testing is ongoing to see if a specific virus is responsible for the outbreak.

Students started becoming sick on their way to a Nationals game Wednesday night. More students became ill during and after the game. Officials say that on the bus trip back from the game, the bus driver detoured to George Washington University Hospital where six students were admitted.

Overnight and Thursday morning, more students became ill and 15 more were hospitalized and treated for dehydration. Of the 40 students who became ill, 21 were hospitalized.

Health officials worked from yesterday into today to determine the cause of the illness. They learned that it was a stomach virus that spread among the students and was not a food-borne illness. They determined that the students did not eat any common things, ruling out food-poisoning as a cause.

All of the students were staying at Piedmont Hall at GMU.

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