The mystery virus which has claimed the lives of more than 100 people around the world is a mutant form of the common cold, say experts.
Sars has infected thousands of people
Tests on samples taken from patients with severe acute respiratory syndrome (Sars) suggest it is a new type of corona virus, which normally causes the common cold.
It has so far infected at least 3,000 people worldwide and killed 106, according to the World Health Organization.
Scientists have suggested the virus, for which there is no known cure, is here to stay and may never be eradicated in some regions.
These latest tests - the most authoritative yet - were carried out by a team of international scientists from eight countries, including Germany and the United States.
SARS: PROBABLE CASES AND DEATHS
China 1,290 cases (55 deaths)
Hong Kong 970 (27)
Singapore 126 (9)
Vietnam 62 (4)
Canada 97 (10)
Thailand 7 (2)
Malaysia 3 (2)
Source: WHO (0600 GMT Friday)
Note: The WHO only records cases and deaths it believes are "probable" Sars - figures from national health authorities may vary.
Their analysis has found that it is not consistent with any other known virus. Genetic tests showed it was "only distantly related" to known corona viruses.
Preliminary results suggest it has never before been seen in some countries, including the US.
The findings, published on the New England Journal of Medicine website, come as the WHO renews its efforts to stop the virus from spreading.
More of its investigators arrived in China, where it first emerged, on Friday to try to gain a better understanding of the disease and draw up better strategies to fight it.
The Chinese government has claimed the outbreak is under control. However, that has been disputed.
Zhong Nanshan, director of the Guangzhou Institute of Respiratory Diseases in Guangdong province, said it was better to describe the illness as "contained".
Nineteen new cases of the illness were reported in the province on Friday.
Figures from the WHO and Chinese Ministry of Health show that more than 1,300 people have been infected with the virus across the country. It has so far claimed the lives of 58 people.
The virus has hit foreign nationals living in China. Two Americans and five other foreigners in Shanghai have been hospitalised with possible Sars, according to government officials and the US Consulate.
Chinese state media reports that nationals from Finland, Canada, Hong Kong, Macau and Taiwan have come down with Sars.
A Finnish official with the International Labour Organisation, Pekka Aro, died of the virus.
On Thursday, friends of a US citizen who died from Sars claimed he had been wheeled, already dead, into an ambulance and sent from Shenzhen to Hong Kong because China's authorities did not want to report that another foreigner had died of the virus in the city.
The US government is recommending that Americans defer non-essential travel to China.
In Hong Kong, officials have said people exposed to Sars will be barred from leaving, amid fears that residents are spreading the disease overseas.
It reported two more deaths and 61 new cases of the virus on Friday - a sharp rise over recent days. The total number of deaths in Hong Kong from Sars now stands at 32.
Officials in Singapore, where nine people have died and 140 have been infected, have now reported seven new cases.
Police there say they are looking for a
Chinese woman who escaped from quarantine earlier this week after
showing symptoms of Sars.
Indonesia and the Philippines each reported their first cases of the virus on Friday.
Malaysia is continuing to block entry of visitors from China and Hong Kong, despite protests from tour operators who fear it will batter the tourism industry.
Meanwhile, health officials have called for passengers who took a flight from
Frankfurt to London on April 2 and one from London to Munich the next day, to
seek medical advice.
They were among seven Lufthansa flights taken by a 48-year-old Chinese man
between March 30 and April 4 who has been diagnosed as suffering from Sars.
The Hong Kong health department said the flights involving the UK were LH 4520
Frankfurt-London on April 2 and LH 4671 London-Munich the next day.
Authorities in the UK have urged passengers on flight LH 4520 from Frankfurt to London on 2 April and flight LH 4671 from London to Munich on 3 April, to seek medical advice. They were among seven Lufthansa flights taken by a 48-year-old Chinese man between March 30 and April 4 who has been diagnosed as suffering from Sars.