The recent revelation that three people in China may be ill with SARS--the disease that emerged and traveled the globe last year, disrupting international travel and commerce¿is fueling fears that the disease will return, perhaps more virulent than ever. Findings published online today by the journal Science describe how the virus has evolved and provide potential clues to developing effective treatments.

Researchers with the Chinese SARS Molecular Epidemiology Consortium analyzed the genomes of 63 samples of the SARS virus collected from patients infected at different times during last year¿s epidemic. The team determined that the two virus genomes that caused the earliest infections in people--11 seemingly independent cases in China¿s Guangdong Province--were identical to strains found in animal hosts. In the middle phase, which began with a so-called super-spreader event in a Guangzhou hospital, a strain missing 29 nucleotides became dominant. The scientists also found changes that caused alterations to amino acids of the virus spike protein, which allows the virus to adhere to host cells. "What we see is the virus fine-tuning itself to enhance its access to a new host: humans," study co-author Chung-I Wu of the University of Chicago explains. "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 late-phase virus was tentatively traced to one patient who became symptomatic on last February 7, according to the report. Noting SARS¿s "remarkable evolutionary tale," the scientists report that their findings support the idea that the virus made the jump from animals to humans, probably multiple times." Notes Wu, "The genetic fingerprints add a whole new layer to our understanding of the course of events in this epidemic."