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."