It is the name being used to describe the serious disease with pneumonia-like symptoms that first emerged in southern China last year but has since gone on to cause deaths in Hong Kong, Vietnam, Singapore, Thailand and Canada. Cases (including three in Britain) have been confirmed in many other countries, mostly among people who had travelled to affected areas of Asia.

What are the symptoms?

High fever, with a temperature of more than 38C, dry cough, breathing difficulties. Chest x-rays can suggest pneumonia.

How many people have been infected?

About 2,300 so far, with nearly 80 deaths - a fatality rate of between 3% and 4%.

How contagious is it?

No one knows. At first, health officials thought it could only be spread through close contact with infected people, especially through droplets exhaled through coughing and sneezing, although other bodily fluids were deemed potentially infective. Many of those who fell ill initially were doctors and nurses who had treated patients with the disease, and there was concern that they had not taken sufficient infection control measures. But evidence from Hong Kong, where nearly a third of the country's 700 or so cases have occurred in the residents of one apartment complex, now suggests it can also spread through shared water or sewage systems.

What causes it?

No one is sure. Currently, the favourite theory is that it is caused by a coronavirus, often the cause of the common cold, which can spread quickly but for which there is no cure. The family of viruses was first identified in chicken in 1937 and can cause serious illness in a range of animals including cows, pigs, rats, cats and dogs. SARS might therefore turn out to be a zoonosis - an animal disease that threatens humans, too.

Traces of paramyxovirus, of the same family of viruses found in measles and mumps, have also been reported by some laboratories. Some scientists speculate that the two viruses have been working together, others that the paramyxovirus is occurring as the result of secondary, unrelated, infections.

How fast does it travel?

The best guess is that the incubation period is brief - between three and six days. That, combined with the speed of modern air travel, means cases are springing up around the world. Some people who have fallen ill seem to start recovering within a week. There is considerable anger that officials in Guangdong ignored the threat originally, failing to alert the WHO when the problem started there in November.

Is there a vaccine or cure?

Neither. Antibiotics and antiviral treatments have been used to combat the symptoms, but until the cause is isolated, work on specific treatments cannot even begin. Travellers returning from south-east Asia who suffer from any of the symptoms within 10 days of their return to the UK should contact their GP or ring NHS Direct on 0845 4647.

How worried should we be?

The WHO is concerned enough to issue its first ever warning to postpone travel to Hong Kong and Guangdong. Usually only wars or other conflicts bring such advice from the organisation. Countries around the world have already been bringing in their own checks on passengers travelling from the region, and the Foreign Office in Britain is expected to follow suit today.

The Department of Health said today that Britons were "strongly advised not to travel" to Hong Kong or Guangdong. The travel warning may be extended to other countries later.