I'm no expert on diseases....
Even now, the source of the 1918 outbreak seems to be unclear, with a host of potential locations.
IF you have an earlier source, that gets carried along (literally) by hundreds of thousands to millions of soldiers on the move, the results could have been absolutely devastating to all sides. The 1918 outbreak hit ages and conditions hard, including those otherwise healthy folks not normally as susceptible by attacking the immune system. Soldiers in the trenches, under extreme physical and mental stress would likely have been hit really hard and would have spread the disease up and down (and across) the trench lines. Conceivably, such an outbreak could have functionally shut down operations on a massive scale.
With limited understanding of the source of the outbreak, might the opposing sides have viewed the disease as a form of biologic warfare?