The influenza A virus subtype H1N1 (initially known as swine influenza virus or as new flu) arrived in Argentina in late April 2009, through air traffic contact with endemic areas, especially Mexico and the United States. The first infection was confirmed on May 7 in a man who had come from Mexico City and had entered the country without symptoms on April 24. The second case was detected on May 22. Towards the end of May infection started in some schools in the northern community of the City of Buenos Aires and its suburbs, the main part of the country affected by the epidemic.