The World Health Organisation raised the level of influenza pandemic alert from phase 4 to phase 5 (out of a possible six levels). This was based on the spread of the new influenza A (H1N1) virus, also sometimes referred to as swine or ‘Mexican’ flu, which has the potential to develop into a pandemic influenza virus. 

http://www.who.int/mediacentre/news/statements/2009/h1n1_20090429/en/index.html