Ft Riley Kansas

Gi's were given vaccinations combining multiple virus/bacteria strains in one shot.
Viruses & bacteria swap RNA, and it happened there.
They then sent sick soldiers to the war, where they got captured by Germans, who caught the flu from prisoners and spread it to the civilian population.
Germany was winning everywhere, until the flu shut down their empire and they surrendered.
Bacteriological warfare successful for the second time in modern history.
First was spreading various pox to the Indian tribes.