World War I may have allowed the emergence of "Spanish" influenza.

The 1918 influenza pandemic caused 40 million deaths, and so dwarfed in mortality and morbidity the preceding pandemic of 1889 and the 1957 and 1968 pandemics. In retrospect, much can be learnt about the source, the possible subterranean spread of virus, and the genetic basis of virulence. The World Health Organization has urged every nation to prepare a pandemic plan for the first global outbreak of the 21st century. We present an appraisal of epidemiological and mortality evidence of early outbreaks of respiratory disease in France and the UK in the years 1915 to 1917. Certain of these earlier focal outbreaks--called epidemic bronchitis rather than influenza--occurred during the winter months when influenza was known to be in circulation, and presented with a particular heliotrope cyanosis that was so prominent in the clinical diagnosis in the world pandemic outbreak of 1918-1919 (the Great Pandemic). The outbreaks in army camps at Etaples in France and Aldershot in the UK in 1916-1917 caused very high mortality in 25-35 year olds. Increased deaths from bronchopneumonia and influenza were also recorded in England. We deduce that early focal outbreaks of influenza-like disease occurred in Europe and on the balance of probability the Great Pandemic was not initiated in Spain in 1918 but in another European country in the winter of 1916 or 1917. We suggest that the pandemic had its origins on the Western Front, and that World War I was a contributor.