Other experts said it was unclear how dangerous the virus is.
"We don't know if this is going to turn into another SARS or if it will disappear into nothing," said Michael Osterholm, a flu expert at the University of Minnesota. He said it was crucial to determine the ratio of severe to mild cases.
Osterholm said it was worrying that at least one person with the disease had died. "You don't die from the common cold," he said. "This gives us reason to think it might be more like SARS," which killed about 10 percent of the people it infected.
Britain's Health Protection Agency and WHO said in statements that the 49-year-old Qatari national became ill on Sept. 3, having previously traveled to Saudi Arabia. He was transferred from Qatar to Britain on Sept. 11 and is being treated in an intensive care unit at a London hospital for problems including kidney failure. Respiratory viruses aren't usually known to cause serious kidney problems.
David Heymann, chairman of the Health Protection Agency, said the new virus didn't appear that similar to SARS.
"It isn't as lethal as SARS and we don't know too much about its transmissibility yet," he said. "If people are getting infected, they aren't getting serious disease."
Heymann said it was unknown whether the virus might mutate to spread more easily in a dangerous form, since viruses mutate constantly as they reproduce. He said none of the health workers involved in treating the Qatari patient had fallen ill.
Saudi officials said they were concerned that the upcoming Hajj pilgrimage next month, which brings millions of people to Saudi Arabia from all over the world, could provide more opportunities for the virus to spread. They advised pilgrims to keep their hands clean and wear masks in crowded places.
The Hajj has previously sparked outbreaks of diseases including the flu, meningitis and polio.