The Spanish flue The Spanish flue was an unusually difficult and deadly form of flue, which spread all over the world in the end of and after the first world war. Its source is unknown. Most of the casualties were young adults, who otherwise were at good health. Normally in flu epidemics there are instead children, old adults and persons with a decreased immune system who are worst affected. The pandemic lasted between Mars 1918 and June 1920 and reached both Arctic and remoted islands in the pacific. Between 50 and 100 million people died, which make the Spanish flue the deadliest pandemic, that lasted such a short time, in the history of humanity.