In experiments conducted at Erasmus University in Rotterdam, Netherlands, scientists infected monkeys with the coronavirus suspected of causing SARS and found that the animals developed the same symptoms of the disease that humans do.
The test was a crucial step in verifying the cause of the disease, which so far has killed 161 people worldwide, mostly in China and Hong Kong, and made 3,293 people ill in 22 countries.
Verifying the cause is important for creating a vaccine, should that be needed, and for refining diagnostic tests to help stop the disease's spread, said Dr Klaus Stohr, a WHO virologist participating in a Geneva conference.
It also will help scientists trace the virus's evolution and possibly determine whether it jumped from animals to humans. Pigs and poultry are being tested to determine how susceptible they are to SARS.
Scientists were almost certain that a new form of coronavirus first isolated from sick patients on March 27 by the University of Hong Kong was the cause of SARS.
But they could not say for sure until they had satisfied what is known as the Koch's postulates four scientific tests that verify whether a bug causes a certain disease.
"The Koch's postulates have been fulfilled, so we can now say for certain that the new coronavirus is the cause of SARS," Stohr said.
Researchers at the University of Hong Kong said a new genetic sequencing of the SARS virus proves conclusively that it came from animals. But, the virus nonetheless is "something that is new to science", university microbiologist Malik Peiris said before the WHO findings were announced during a meeting of scientists from around the world working on SARS.
Asked about the possibility that the virus was man-made, Peiris said there was no chance of that. "That whole genome is essentially new," he said. "Nature has been the terrorist throwing up this virus."
Although experts believed the new coronavirus discovered by Peiris was the main cause of SARS, it has remained unclear whether infection with a second type of virus the human metapneumovirus, which belongs to the paramyxovirus family makes the illness worse.
But the tests on monkeys showed that the second virus does not play a major role, said Albert Osterhaus, a virologist at the lab in Rotterdam.
Researchers at a Singapore government-run institute reportedly are almost ready to begin trials of a test to detect the presence of SARS in a patient's blood before the onset of symptoms.