During the  influenza pandemic, about one-third of the world was infected and at least 50 million died.
But why was the pandemic so deadly?
Research finds clues in climate. In a paper published [September 15] in the journal GeoHealth, scientists analyzed the effects of an extreme weather anomaly they said set the stage for increased casualties during World War I and the spread and intensity of the flu afterward.
Using an ice core from the Alps and other climate records, they found evidence of an abnormal influx of cold air into Europe between 1914 and 1919. As a result, temperatures plunged and rain flooded battlefields.
The war contributed to the terrible weather, they said: The dust and explosives generated in battle probably cooled the local atmosphere and prompted precipitation.
The climate also changed bird migration.
Mallard ducks, the main carriers of H1N1, stayed put instead of migrating from Western Europe to Russia, the researchers said. The ducks probably infected water that humans then came into contact with.
The pandemic was also aided by unsanitary conditions caused by the war and the use of chemical weapons, which have been implicated in the mutation of the virus to its most severe form.
The researchers found that spikes in deaths during the war usually followed cold temperatures and heavy rain. They said that the chilly temperatures could have contributed to co-infection with pneumonia, which made the death toll even higher.