Pandemic influenza has emerged 1-3 times each century. The pandemic in 1918, or the "Spanish flu" was caused by a novel influenza strain which caused the death of 21-50 million people world wide. Descriptions of the epidemic in Iceland give a detailed account on how and when the virus was introduced to the population of Reykjavik, the capital of Iceland, by the crew of two ships, "Botnía". and "Willemoes" on October 19th 1918. The spread of the illness was extremely rapid and peaked 3 weeks later. It caused significant morbidity and mortality among inhabitants of the southern and western part of Iceland. Within 6 weeks, close to 500 individuals had died, thereof more than 50% in Reykjavik. The attack rate in Reykjavik was at least 63% and the case fatality proportion was close to 2.6%. The age-specific mortality was highest among young children, people 20-40 years of age and the elderly. In addition, pregnant women had extremely poor prognosis (37% case fatality). Attempts to halt the spread of the epidemic to the northern and eastern parts of the island were successful. By identifying the individuals who died from the Spanish flu using historical data, it has recently been shown that genetic factors probably did not play a major role in the pathogenesis of fatal cases. These historical data can be used to assist in planning for new pandemics of influenza, which are believed to be inevitable.