Summary: In 1918, the world was still in the throes of the Great War, the deadliest conflict in human history at that point, but while World War I would be a catastrophic war surpassed only by World War II, an unprecedented influenza outbreak that same year inflicted casualties that would make both wars pale in comparison. An illness, or more likely a collection of illnesses, Spanish influenza quickly spread across the world and may have killed upwards of 100 million people, decimating populations across developed nations and possibly wiping out as much as 5% of the world's population.