Influenza, or flu, has caused more deaths than any single disease outbreak since the 14th-century bubonic plague. Twenty to fifty million people worldwide died from the 1918 Spanish Flu, which was an H1N1 strain similar to the 2009 pandemic. ‘Influenza’ considers the attempts to control transmission of flu by reducing encounters, by reducing compatibility (through vaccination), or ideally by a combination of both. The unique evolutionary potential of flu means that both vaccinated people and unvaccinated people who contract flu naturally tend to lose their immunity after a few years. General principles of evolutionary biology, as well as intriguing particulars of flu evolution, are introduced. The politics of research and risk assessment are also discussed.