Evidence mounted today that a new strain of a virus linked to the common cold is the main cause of the SARS virus that has killed 101 people and infected about 2,800 around the world.
In newly published research about the deadly infection, scientists in Hong Kong who studied 50 patients with Severe Acute Respiratory Syndrome (SARS) from five outbreak clusters said it was a new member of the coronavirus family.
"We have provided evidence that a virus in the coronavirus family is the causal agent of SARS," Mr Malik Peiris of the University of Hong Kong said in a report published online by The Lancetmedical journal.
"However, it remains possible that other viruses act as opportunistic secondary invaders to enhance the disease progression..," he added.
After identifying the new type of coronavirus from two SARS patients, Mr Peiris and his colleagues looked for evidence of it in others with the illness and in healthy individuals.
They found virus activity in 90 percent of the patients but not in any of the healthy control group.
Ms Ann Falsey and Mr Edward Walsh, of the University of Rochester in New York, said the findings strengthen the case that it is a novel coronavirus.
"As other pathogens...are identified in SARS patients, it will be important to use control groups to determine their role in causality or as cofactors for severe disease," they said in a commentary on the research.
A spokesman for the World Health Organisation (WHO) said last week that the UN agency was 99 percent sure it belonged to that family.
The WHO, governments and national health authorities have issued travel advisories and taken precautions to limit the spread of the illness which is characterised by a dry cough, high fever and breathing difficulty.
Scientists believe it is spread through droplets by sneezing and coughing or by direct contact. Hong Kong health officials said on Tuesday cockroaches may have spread the virus in an apartment complex in the densely populated city.
If they are right, it would be an alarming development because it would be even more difficult to contain. Hong Kong has the highest number of infections of SARS after China.
A WHO official has also expressed fears that SARS could be carried by people without symptoms. Mr Peiris said airborne spread of the virus cannot be ruled out.