The 2009 flu pandemic is a global outbreak of a new strain of influenza A virus subtype H1N1, first identified in April 2009, termed Pandemic H1N1/09 virus by the World Health Organization and colloquially called swine flu. The outbreak was first observed in Mexico, and quickly spread globally. On the 11 June 2009, the WHO declared the outbreak to be a pandemic.The overwhelming majority of patients experience mild symptoms",but some persons are in higher risk groups, such as those with asthma, diabetes,obesity, heart disease, or who are pregnant or have a weakened immune system.In the rare severe cases, around 3 5 days after symptoms manifest, the sufferer's condition declines quickly, often to the point of respiratory failure.The virus reached the United Kingdom in April 2009.The first cases were confirmed on 27 April 2009 in passengers returning from Mexico. The first case of person to person transmission within the UK was announced on 1 May 2009. In the UK, 5-14 year olds are the age group predominantly affected.