It is not generally appreciated that descendents of the H1N1influenza A virus that caused the catastrophic and historicpandemic of 1918–1919 have persisted in humans for morethan 90 years and have continued to contribute their genes tonew viruses, causing new pandemics, epidemics, and epizootics(see table). The current international pandemic caused by anovel influenza A (H1N1) virus derived from two unrelated swineviruses, one of them a derivative of the 1918 human virus,3adds to the complexity surrounding this persistent progenitorvirus, its descendants, and its several lineages (see diagram).