The World Health Organization has declared that a mysterious strain of atypical pneumonia thought to have sickened hundreds of people in Asia is a new disease. In Hong Kong the number of people suffering from the illness has almost doubled overnight.

World Health Organization officials in Beijing said Monday that "a new communicable disease" is causing atypical pneumonia outbreaks in southern China, Hong Kong and Hanoi.

Doctors say the disease, known as Severe Acute Respiratory Syndrome, is likely to be a virus and not bacteria. At least four hundred people in Asia have symptoms resembling the disease, but doctors have not determined if they are all linked.

Peter Cordingley, a WHO spokesman in Manila, says the disease can cause a rapid deterioration of the respiratory system. "This infection can be passed only by direct and sustained contact."

Ninety-five people in Hong Kong are being treated with serious flu-like symptoms - almost double the number of sufferers reported on Sunday.

At least seven people are thought to have died from atypical pneumonia in Asia in the past few months.