 Annual influenza epidemics are estimated to affect 5 – 15 % of the global population . Although most cases are mild , these epidemics still cause severe illness in 3 – 5 million people and 250 @,@ 000 – 500 @,@ 000 deaths worldwide . On average 41 @,@ 400 people die of influenza @-@ related illnesses each year in the United States , based on data collected between 1979 and 2001 . In industrialised countries , severe illness and deaths occur mainly in the high @-@ risk populations of infants , the elderly and chronically ill patients , although the H1N1 flu outbreak ( like the 1918 Spanish flu ) differs in its tendency to affect younger , healthier people . 
