From spring 2009 onwards, a new influenza A virus of the H1N1 subtype started to circulate among humans in Mexico. Tens of thousands were infected before the virus spread to Texas and California, where it was first identified as a new human virus that probably had originated from pigs. The virus had initially not been identified in pigs, although it has spilled back from humans to pigs, and it is not clear whether the reassortment had indeed taken place in this species. Since April 2009, the virus spread efficiently among people in the USA and Canada. By the middle of June it was already identified in more than 80 countries, to eventually spread world-wide in the following months. Transportation via air traffic has largely contributed to this rapid spread. In several countries little sustained human transmission took place initially. In some countries initial societal as well as targeted medical interventions to prevent its spread may have delayed the spread even further. These included intensive surveillance activities among travelers from Mexico and other areas where sustained transmission did take place initially, and antiviral treatment of infected individuals and their contacts.