Experts say they have now confirmed the identity of a virus which may have killed more than 50 people worldwide.
The US Centers for Disease Control says that a Corona virus - possibly a strain never before seen in humans - is to blame.
A Chinese mother and daughter
Other types of Corona virus can cause the common cold.
The results came as increasing concern over the infection prompted the closure of all Hong Kong schools for more than a week.
The Hong Kong Government has ordered people who believe they have been exposed to the virus to stay away from work - or be fined or jailed.
The Chinese Government revealed on Wednesday that 34 people had so far died in an outbreak of respiratory illness - mostly in the southern province of Guangdong.
The World Health Organisation says that the Guangdong infection has strong similarities to the SARS illness which has killed 19 and infected hundreds more elsewhere in the world.
However, this link has not yet been definitively proved.
SARS so far - cases (deaths)
Hong Kong - 357 (11)
Singapore - 74 (2)
Vietnam - 59 (4)
Canada - 33 (3)
Britain - 3 (0)
US - 40 (0)
China - 792 (34)
(Note: China cases not confirmed as SARS)
Dr Malik Peiris from Hong Kong University told a news conference: "It appears to us that the Corona virus is the primary cause of the disease...it is possible of course that other viruses might also infect the same patient and might increase the severity of the disease."
He suggested that the SARS strain might have originated from an animal virus which mutated to infect humans.
The south China region has historically been associated with animal viruses "jumping" to humans.
The identification of the virus could allow doctors to develop a test to rapidly identify genuine SARS sufferers.
All Hong Kong schools have been ordered to shut until at least April 6 to reduce the spread of the disease, it was announced on Thursday.
More than 350 people in Hong Kong are now known to have been infected, and 11 have died.
The authorities there have imposed a quarantine law.
People who think they may have come into contact with a person known to be infected have to report for assessment at a clinic - and must stay away from work for 10 days.
Singapore has reported its first death from the illness, and has also announced the closure of all schools until the same date.
Likely culprit: Coronavirus
Almost 500 people outside China have been infected with the illness, according to most recent World Health Organisation figures.
New suspected cases have also emerged in Canada, where three people have already died from SARS.
In Singapore, the Ministry of Health has invoked special powers to keep 740 people under home quarantine in an attempt to try to halt further spread of the disease.
Bookings on Singapore Airlines have plummeted since the first cases emerged, tourism authorities