Another 20 British soldiers in Afghanistan have come down with symptoms of a mysterious disease, and one of the men was so sick he was to be evacuated Friday, a British military spokesman said. 

Seven British soldiers have already been evacuated — six to Britain and one to a U.S. military hospital in Germany — since the outbreak began Sunday. 

Lt. Col. Ben Curry said two of the 20 soldiers were isolated after displaying symptoms of the unknown disease, but it was unclear what it was. 

British officials discovered the possible new cases Thursday after a senior medical officer ordered a check of all British troops deployed at Bagram air base "in order to identify anyone who displayed the symptoms of diarrhea and vomiting." 

Curry said the 20 personnel had reported the symptoms during the check. The most serious case was to be evacuated to Britain Friday, while two others showing serious symptoms of the infection were isolated and treated. The remaining 17 had shown milder symptoms and were still in their tents. 

Some 334 British troops, most at 34 Field Hospital, have been put under quarantine. Only the most serious of the 20 possible new cases was put under quarantine. 

Soldiers have shown symptoms similar to an intestinal fever. Eight patients have recovered so far, but are still under quarantine. 

Military officials have rejected the possibility of a biological terrorist attack. 

About 5,000 soldiers from some 10 countries are in Bagram. None of the about 2,700 American troops deployed there are thought to have contracted the illness, said U.S. military spokesman Maj. Bryan Hilferty.