Within days of confirming fresh cases of the new swine flu in California and Texas last week, scientists at the Centers for Disease Control (CDC), in Atlanta, had sequenced the entire genome of the virus. By Monday, much of that genomic information was loaded into a publicly available database, allowing the world's scientists to begin searching for clues to the origins of the mysterious virus and the severity of the threat it poses.

The World Health Organization (WHO) raised its alert to level 5 on Wednesday, signaling that a pandemic is imminent, and making the search for new information even more urgent. "The biggest question now is, how severe will the pandemic be?" said Margaret Chan, WHO director-general, at a press conference on Wednesday evening. Currently, symptoms appear to range from mild to lethal.

Researchers hope that the genome will shed light on the pathogen's power, as well as on its origins. Clusters of cases around the world already suggest that it is highly transmissible between humans. But it's not yet clear how deadly the virus is--namely, whether it's significantly more lethal than the seasonal flu virus, which kills an average of 36,000 people in the United States each year.

It's also unclear whether the high mortality rates of the swine flu observed to date in Mexico, as compared to in the United States, reflect differences in the virus itself, or are linked to environmental factors such as differences in treatment, or an underestimate of infection rates in Mexico. There have been about 160 suspected swine flu deaths among approximately 2,500 people treated with severe pneumonia in Mexico since the outbreak began (Mexico's National Institute for Public Health reports 49 confirmed infections and seven deaths). As of Wednesday afternoon, the CDC reported 91 confirmed cases in the United States and one death.

"Right now, we don't have any evidence of a difference in virulence, but that's one possibility we're looking at," said Richard Besser, acting director of the CDC, at a press conference on Tuesday. As of Wednesday afternoon, sequence information from virus samples isolated from patients in Mexico was not available through the public database, but there was data from samples collected in California, Texas, New York, Kansas, Ohio, and Germany. Neither the WHO nor the CDC could explain why.

"Given that this is still a rapidly evolving story, we need to get a handle on the first gene sequences from California, and hopefully from Mexico and others, to see how this virus has evolved," says Ram Sasisekharan, a professor of biological engineering at MIT. "Where does it really come from? And can we explain the observed differences in what has happened in Mexico, which seems much more severe than what has happened in the U.S.?"