In WW1 they were using gasses it could be possible a bio agent was released in 1917 through some gas

In the winter of 1917, several hundred British soldiers collapsed with influenza-like symptoms and medics at Étaples recorded 156 deaths. At the time, the epidemic was labelled “purulent bronchitis” because of the yellow pus that oozed from the larger airways of the lungs at autopsy (some medics thought it resembled the lung damage from phosgene gas).

Another prominent feature was cyanosis, a distinctive purple-blue discolouration of the lips, ears and cheeks, caused by the loss of oxygen to the heart. Cyanosis was also a hallmark of the pneumonias associated with the Spanish flu – an observation that persuaded doctors writing in the Lancet in 1919 that it and purulent bronchitis had been “fundamentally the same condition”.