Presence of co-morbidities or risk factors for severe disease Several populations are at risk for severe disease from seasonal as well as 2009 H1N1 infection [4] , including individuals who have pre-existing illnesses or medical conditions, pregnant women, immunosuppressed individuals (either through treatment, HIV infection, or as a result of a pre-existing immunosuppressive disorder) and children aged 0-4 years. Medical conditions associated with an increased risk of severe disease include chronic respiratory disorders (for example, asthma, bronchitis, chronic obstructive pulmonary disease [COPD] and cystic fibrosis), neuromuscular disorders (for example, cerebral palsy, myasthenia gravis and muscular dystrophy), metabolic diseases (for example, diabetes) and chronic renal, heart or liver disorders [3] . Factors such as obesity and hypertension are not normally associated with severe disease from seasonal influenza but have been suggested as risk factors for severe disease from 2009 H1N1 in some studies [5] [6] [7] .