From 1918 to 1919, an outbreak of influenza ravaged Europe and North America. The outbreak was a pandemic; that is, individuals in a vast geographic area were affected. In the case of this particular influenza outbreak, people were infected around the world.

The pandemic killed more people, some 20 to 40 million, than had been killed in the just-ending Great War (now known as World War I). Indeed, the pandemic is still the most devastating microbiological event in the recorded history of the world. At the height of the epidemic, fully one-fifth of the world's population was infected with the virus.

The disease first arose in the fall of 1918, as World War I was nearing its end. The genesis of the disease caused by the strain of influenza virus may have been the deplorable conditions experienced by soldiers in the trenches that...