International health officials are warning that a deadly avian influenza virus may soon spread rapidly, overwhelming unprepared health systems in rich and poor countries alike. If the virus mutates to become easily transmittable among humans, the death toll of the resulting global pandemic could number in the millions. Since it first emerged in 1997, avian influenza has become deadlier and more resilient. It has infected 109 people and killed 59 of them. If the virus becomes capable of human-to-human transmission and retains its extraordinary potency, humanity could face a pandemic unlike any ever witnessed. If an influenza pandemic struck today, borders would close, the global economy would shut down, international vaccine supplies and health-care systems would be overwhelmed, and panic would reign. To limit the fallout, the industrialized world must create a detailed response strategy involving the public and private sectors.