The speed at which 2009 A/H1N1 spread during the early phases of this pandemic is striking. It was detected in four continents within three weeks after Mexican authorities first reported it. In contrast, the 1918 Spanish flu took 3 years to circle the globe [23] . Our analyses of air-travel data support the WHO's decision to recommend against closing all air travel from Mexico, since the virus most likely had already spread to several other countries by the time it was first reported to be widespread in Mexico on April 29 th . In particular, cases had already been detected in the United States, which is a major hub for connecting flights [24] .