2018 was the 100th anniversary of one of the largest and most devastating flu pandemics in modern history in which more people died than in all of World War I. This course describes the current and historical impact of influenza, seasonal and pandemic. It includes influenza epidemiology, virus types and subtypes, how influenza viruses drift and shift, and a review of the worldwide impact of the 1918–1919 flu pandemic. We also discuss the goal of universal vaccination, diagnosis, and treatment, and the composition of the 2021 influenza vaccines.