The pandemic caused by the 2009 influenza A(H1N1) virus, which began in Mexico, spread to every continent within 11 weeks. Its progression took the form of single-peak epidemic waves in temperate-climate countries in both the Northern and Southern Hemispheres. While the magnitude of these waves remains difficult to estimate, their duration—as observed particularly in the Southern Hemisphere—was approximately 8 to 12 weeks, comparable to that of seasonal influenza epidemics. The epidemic peak, reached in 4 to 6 weeks, was narrower and 2 to 3 times higher than that observed during recent seasonal influenza epidemics, which tended to overwhelm intensive care units. While the highest incidence rates are found in the under-25 age group, the highest mortality rate is found among adults aged 20 to 50. During seasonal influenza epidemics, mortality is highest among those over 65. The case fatality rate is estimated to be between 1 in 1,000 and 1 in 10,000, comparable to that of seasonal influenza. It is difficult to predict how the outbreak and the virus will evolve in the coming months, and many questions remain unanswered. (R.A.)