The origins of Sars have been made clearer by a genetic study that traces it back to wild animals in China.
The coronavirus: Mutated rapidly
As scientists worry about the potential for bird flu to one day become a major killer, the study shows how Sars "jumped" to humans a year ago.
More than 800 deaths and 8,000 cases resulted from Sars outbreaks in Hong Kong, China and Canada.
Researcher Dr Chung-I-Wu, from Chicago University, said it was "disturbing" to watch it gradually gain virulence.
The current Avian influenza outbreak in the far East is related to Sars only in demonstrating how so-called zoonotic diseases in animals can cross to become a threat to human health.
Although not easily passed from human to human, Sars caused a life-threatening respiratory infection in many cases, and experts are braced for its return this winter.
Researchers are now convinced that Sars made that jump in late 2002 in the Pearl River Delta in the Guangdong Province of China.
They compared the genomes of viruses isolated early in the emergence of Sars, and compared them to isolates from later victims of the virus.
This showed how the coronavirus which causes the illness adapted to become better at living in humans.
Two "genotypes" dominated the early phase of the epidemic - these were roughly comparable with coronavirus isolates taken from animals in a market in Shenzen.
In the middle phase of the outbreak - including the "super-spreader" event at a hospital in Guangzhou in January 2003 - during which case numbers multiplied, the genetic makeup of the virus had already shifted.
Different versions of the coronavirus predominated - with one in the hospital and another emerging in Hong Kong.
Within a month, another genotype had emerged, which persisted to the end of the outbreak in August.
When scientists examined levels of gene mutations in one particular gene - called spike - which was thought to be involved in the ability of the virus to enter cells, it was found to be undergoing rapid mutations at the start of the outbreak, which slowed down after it had refined its ability to infect humans.
Dr Chung I-Wu said: "What we see is the virus fine-tuning itself to enhance its access to a new host - humans.
Sars has caused widespread fear
"This is a disturbing process to watch, as the virus improves itself under selective pressure, learning to spread from person to person, then sticking with the version that is most effective.
"The genetic fingerprints add a whole new layer to our understanding of the course of events in this epidemic."