Scientists said on Thursday they had identified the virus behind the respiratory illness spreading worldwide.
The coronavirus, a relative of one of the many viruses that cause the common cold, is, as suspected, new to humans, two research teams reported in the New England Journal of Medicine.
The finding means that doctors can now concentrate on developing a simple test for the virus that will tell them right away whether a patient has Severe Acute Respiratory Syndrome.
The US Centre for Disease Control and Prevention has three such tests but says they are not suitable for everyday use.
In one of the studies reported on Thursday, the CDC's Dr Larry Anderson and colleagues tested samples from patients in six countries with SARS.
"Nineteen patients with SARS have been identified as infected with the new coronavirus. All have direct or indirect links to the SARS outbreak in Hong Kong and Guangdong province, China," the researchers said in their report released early by the journal.
"A coronavirus with identical (genetic) sequences has also been detected in a patient with SARS in Canada."
They said the virus should be named after Dr Carlo Urbani, the World Health Organisation doctor who died of SARS last month after treating one of the first patients infected with the virus in Vietnam.
"Because of the death of Dr Carlo Urbani during the investigation of the initial SARS epidemic, we propose that the virus be named Urbani SARS-associated coronavirus," they wrote.
SARS is marked by a high fever, dry cough and other flu-like symptoms but it progresses to pneumonia. Some patients must be put on respirators to help their lungs function.
About four per cent of patients with SARS die.
SARS, which was spread around the world by travellers, has killed an estimated 110 people and infected more than 3,000. But authorities in the US and other countries believe they have the infection under control.
In China, Hong Kong and Singapore, areas hardest hit by the virus, the picture is less clear.
The CDC, WHO and doctors in affected areas, eager to find the root of the mystery disease, tested for the usual suspects, such as influenza and other known bacterial and viral causes of pneumonia, which turned out negative.
At first a virus related to measles, mumps and some other more exotic diseases emerged as the cause of SARS, but scientists later ruled that out.
It is possible that the virus, called a paramyxovirus, or other microbes may help make patients more ill or make them more likely to transmit SARS, Anderson's team said.
The lung damage seen in patients who died of SARS looks more like the damage done by measles, respiratory syncytial virus and some other diseases, and not like the damage done by other coronaviruses, they said.
It is possible the damage is caused by the body's immune response. When the immune system attacks a bacterial or viral infection, it sometimes kills healthy cells along with the microbes.
The CDC team is working to sequence the DNA of the virus, which will give a better idea of what it is and where it originates. But it does not look like anything they have seen before in animals or people.
"Preliminary studies suggest that this virus may never before have infected the US population," they wrote.
"Certainly, it has not circulated widely in humans," they wrote. "Presumably, this virus originated in animals and mutated or recombined in a fashion that permitted it to infect, cause disease, and pass from person to person."
In a second study Dr Christian Drosten of the Bernhard Nocht Institute for Tropical Medicine in Hamburg and colleagues across Germany, France and the Netherlands also pointed to coronavirus.
They tested samples from 18 SARS patients in Hanoi and 21 healthy people who had been in contact with the patients.
All of the patients with severe SARS had the virus, while none of the healthy people had it. Of those with suspected SARS, the virus could be found in 23 per cent.
