Eighteen British soldiers at the Bagram air base have been sickened with a mysterious, contagious illness that resembles meningitis, and 350 military personnel have been quarantined to prevent it from spreading.

Brig. Rodger Lane, spokesman for the Royal Marines, said two of the 18 victims have been airlifted out of Afghanistan in very serious condition, and their next of kin have been notified.

One soldier was evacuated to the United Kingdom; the other was taken to a U.S. military hospital in Germany.

Lane said the symptoms — high fever accompanied by severe diarrhea and vomiting — began three days ago, and that all the victims were military medical personnel.

He said there is no indication that the illness has spread, but precautions are being taken, including a quarantine implemented at the 34th Field Hospital, about 30 miles north of Kabul.

"We believe it's some kind of enteric (intestinal) fever, but we have yet to go and establish what exactly it is," Lane said.

Lane said the illness resembles meningitis, but medics do not believe that is the culprit.

Lane said the area around the 34th Field Hospital has been isolated, and military police have been deployed to prevent trespassers. There are no Americans within half a mile of the air base.

About 1,700 British troops are currently deployed at Bagram and have been involved in two military operations in eastern Afghanistan since arriving in the country last month.

Lane said, however, that no hospital personnel had left the base since they arrived. The hospital has treated almost exclusively British troops, though one Afghan was treated two weeks ago, he said.

Earlier this month, three British Royal Marines were evacuated to Bagram during an operation in mountainous eastern Afghanistan. Two were diagnosed with altitude sickness and one had dysentery.

Fox News' William LaJeunesse and the Associated Press contributed to this story.