The clinical manifestations of the ‘Swine flu’ were relatively mild by the end of May 2009 and reminiscent of what is normally seen in seasonal influenza. Yet, a higher percentage of gastro-intestinal symptoms was observed and many severely affected people were in relatively young age categories with an overall peak between 25 and 50 years. Although initial figures from Mexico were hard to interpret, case fatality rates of the ‘Swine flu’ also proved to be relatively low. However, due to the relatively young age of the severe and fatal cases the overall impact still seems to have been huge. The transmissibility of the new virus proved at least comparable with that of seasonal influenza viruses. On the basis of the transmissibility leading to the worldwide spread of the virus in humans, WHO declared that the world was in the pandemic phase by 11 June 2009.