No new cases were reported Thursday at the main allied air base at Bagram, north of the capital, where the illness was first reported Sunday, said British military spokesman Lt. Col. Ben Curry.
"We were encouraged by the fact that there have been no additional cases," Curry said. "We still cannot categorically identify the source or nature of the infection."
Symptoms were similar to an intestinal fever, or "enteric febrile illness," which he said "can in some cases show meningitis-like symptoms."
British soldiers have reported symptoms including fever, diarrhea and vomiting, but Curry said it is probably not meningitis. Another spokesman, Maj. Jeff Moulton, said the intensity of symptoms differed widely.
"Some people have had mild upset stomachs while others have been on the verge of unconsciousness," Moulton said.
The two worst of the 18 cases were flown out of the country on Monday and Tuesday — one to Britain and another to a U.S. military hospital in Germany. Curry said the two were "stabilizing."
Six more soldiers were evacuated by plane Thursday morning, but officials said they were not seriously ill. Six others were being treated by medical staff wearing masks, gloves and aprons at 34 Field Hospital, where 333 people were put under quarantine to prevent any spread of the disease, Curry said.
Eight of the 18 patients who fell sick have recovered. They were discharged from the hospital, but are still under quarantine, Curry said.
Military officials have rejected the possibility of bio-terrorism.
"There's absolutely no feeling at all that this is linked to terrorists or chemical or biological weapons," Moulton said. "Investigations are still going on, but the suspicion is that it's probably linked to a food source."
British troops had been eating both locally purchased food as well as military rations. Now, all those under quarantine were eating military rations as "a force protection measure," Curry said
He said the quarantine, which began Wednesday, would likely last another two days if no new cases are reported.
Those quarantined include hospital staff and "supporting units" of the 1,700-man British mission, Curry said, giving no other details. Military police have been deployed to prevent people from trespassing into the area.
Bagram hosts about 5,000 soldiers from about 10 countries who are participating in the coalition against terror. American troops number about 2,700, but none of them was thought to have contracted the illness, said U.S. military spokesman Maj. Bryan Hilferty.
"We have one American patient last night who has somewhat similar symptoms, but we don't think it's the same thing," Hilferty said. "But of course we are watching everyone very carefully."
Curry said the commanding officer of 34 Field Regiment, who was himself under quarantine, was leading the investigation at Bagram, while doctors in Britain were examining patients who had already arrived there.
None of the soldiers who fell ill had been out on field operations. The hospital has treated almost exclusively British troops, though one Afghan was treated two weeks ago, officials said.
British soldiers have taken a range of precautions to avoid falling sick, including using malarial prophylactics, dipping their uniforms in insecticides and drinking bottled water.
Curry said British soldiers who needed emergency care could be sent to an American field surgeon at Bagram and a German hospital in Kabul.
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.
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