Influenza epidemiology involves studying the distribution, determinants, and impact of influenza viruses within populations. Seasonal influenza epidemics occur annually, typically in the fall and winter months in temperate regions. Pandemics, caused by novel influenza A viruses, occur less frequently but can have significant global impacts. The 1918 Spanish Flu, the 1957 Asian Flu, the 1968 Hong Kong Flu, and the 2009 H1N1 pandemic are notable examples.
