Scientists are only now getting a handle on the details of the outbreak, so governments have taken a path of caution.
In recent days, the world has taken far-reaching steps to brace itself against the swine flu: The World Health Organization has raised its pandemic alert level to 5 on a six-point scale, the United States secretary of Homeland Security has declared a public-health emergency, and federal officials have released a stockpile of vaccines to state and local public-health providers.
The US has recorded 109 confirmed cases and one death related to this new variant of the flu. Yet in a typical flu season, the Centers for Disease Control and Prevention (CDC) estimate, some 36,000 people die in America alone because of complications from the illness.
Given the relatively small footprint of the disease so far, why has the US and the world community responded so overwhelmingly?
The answer: caution in the face of what scientists and public-health officials say they don't know about the virus.
In situations like these, public-heath officials walk a fine line between taking what they see as prudent precautions and triggering what some specialists call excessive fear.
Year to year, scientists see small genetic variations in the flu virus, but not enough to raise doubts about individuals' immunity to them or the effectiveness of existing vaccines. But the new virus raises questions that scientists have not yet been able to answer fully.
"We're seeing a big change" in the virus's genetic makeup, says Theodore Cohen, a communicable-disease specialist at Brigham and Women's Hospital in Boston. Until these changes can be understood, the response is discretion.