Health experts are trying to identify a new Sars-like virus that is so far known to have infected two people, one of whom is receiving intensive care in a London hospital.
A 49-year-old Qatari man was transported by air ambulance from Doha almost two weeks ago. The other person, a 60-year-old man, died in Saudi Arabia, where the Qatari had recently travelled before falling ill.
The UK's Health Protection Agency (HPA) is drawing up advice to help healthcare workers recognise in others the symptoms that occurred in the two men. These include fever, cough, shortness of breath and other breathing difficulties.
It is not clear at present whether the two confirmed cases are typical or the virus could be circulating more widely, but only rarely causing severe illness.
John Watson, head of respiratory diseases at the HPA, said: "The HPA is providing advice to healthcare workers to ensure the patient under investigation is being treated appropriately."
"In the light of the severity of the illness that has been identified in the two confirmed cases, immediate steps have been taken to ensure that people who have been in contact with the UK case have not been infected, and there is no evidence to suggest they have."
The Sars crisis of 2003, which seems to have started in southern China the previous year but was not identified as a serious problem until an outbreak in Vietnam, quickly become a multicountry epidemic. There were an estimated 8,000 cases, including some in the UK, and 750 deaths.
The new virus is different from any previously identified, the HPA says. Preliminary inquiries are said to have revealed no evidence of illness in contacts of the two cases, including healthcare workers.