That summer and autumn more than 1.5 million American soldiers crossed the Atlantic for war. A few of the men from Kansas brought onboard a silent passenger: the flu virus. Almost immediately, ships became floating hospitals instead of battleships. Some doughboys never made it to the European battlefields. Instead, they died at sea. The survivors carried the disease to the front lines of the war.
The virus that causes influenza is a changing creature, and the circumstances of war created an even greater opportunity for the virus to remake itself. As it traveled and passed through one body to another, meeting different immunological makeups and adapting to overcome each new environment, the microbe mutated and became viciously contagious and more deadly. In the mud and the rain of the war's front lines, a trenchmate's sneeze carrying virus particles could contain more killing power than an enemy's bullet.
The flu ignored hostile borders and traveled into Italy, Germany and France. Soon the British were laid low with the new illness as well, and the Spanish was especially devastated. In fact, the pandemic became known as Spanish Influenza, not because it originated in Spain but because so many Spaniards caught the disease — an estimated 8 million.