LONDON, May 17: The mysterious illness raging among British troops in Afghanistan has reached the UK mainland after a doctor who was treating the sick troops flew back and spent three days without knowing that he had contracted the disease.

He is the first victim of this potentially deadly disease to have shown symptoms in the UK, and he might already have spread the disease to several others in Britain. He has been put under quarantine.

The most dangerous aspect of the disease, according to experts, is that it is spreading through the air without any body contact, and there are great chances that the illness could outbreak in the whole of the United Kingdom.

Seven of the 26 British soldiers who have contracted the disease in Afghanistan have been airlifted to Britain for treatment and one to a US base in Germany. Two of them are in a very serious condition.

The case has confirmed the worst fears of British commanders, which is that the disease is airborne, rather than caught from local food, and has caused concern about infection among the troops because of the cramped conditions of the 9,000 based in Afghanistan.

At Bagram, a further 22 cases have joined the six patients still affected, and the bug has spread beyond the 34 Field Hospital Unit, where the original cases were discovered.

It is believed that at least 40 British soldiers are affected with the illness. The new outbreak dashes hopes that the disease has been contained by the quarantine of the field hospital unit.

Despite the area being quarantined, the bug has spread among other British troops among the 1,700-strong 45 Royal Marines Commando task force. British Army spokesman Lieutenant Colonel Ben Curry refused to say which British units were involved, but said three soldiers were in isolation and 17 more had symptoms.

But he said the new cases all came from two camps, Camp Gibraltar, home to the main Marines force; and Camp Hacienda, home to Special Forces units. The British military experts have so far ruled out the possibility that the disease was actually a biological warfare attack from al Qaeda.

Colonel Curry said: “There’s absolutely no reason to suggest that there is a bio-terrorist threat.”

Now the scientists are fighting the disease at two fronts: stopping its outbreak within the UK and also controlling it from spreading to other peacekeeping soldiers in Afghanistan.

Dr Paul Clarke, a former military medical consultant and an infectious diseases expert, said: “This is most likely to be a severe viral infection. This kind of thing is common in countries where there are poor sewage systems.”