War causes many people to live in unsanitary conditions for long periods of time and as a result, diseases are widespread. It got to a point where diseases were taking more lives than the actual combat, so America’s military leaders took action.
By the 1930s there had been a rise in antibiotics to treats wounds and infections of a general population. However, caring for diseases is costly and inefficient in combat zones so military leaders sought a preventative method for deterring illness of their troops. During the first World War, soldiers returning to America from Europe brought back sickness and caused the influenza pandemic in 1918, which killed more Americans than the war did.
To ensure this would never happen again the American Military teamed up with medical researchers to cultivate the first flu vaccine. This research was not conducted for profit but as a service for the greater good of the world. As many as ten vaccinations were created during WWII, although the influenza formulas require adjusting each you as the strain evolves, a practice which still continues to this day.
“For instance, botulinum toxoid was mass-produced prior to D-Day in response to (faulty) intelligence that Germany had loaded V-1 bombs with the toxin that causes botulism. Japanese encephalitis vaccine was developed in anticipation of an Allied land invasion of Japan.”
This video gives a look at the severity of diseases caused by the first World War and the influenza pandemic that gave rise to the preventative medicine we know today.