To quote my grandfather's memoirs:
The enemy had gained possession of the high ground in the area during the November battle and even such small bumps as Hill 60 were in their hands. Entrenched on the forward slopes, they could observe and frequently enfilade the British lines, and, even more uncomfortably, could drain their positions into our lines. The never-ending task of trench drainage became a major scourge to the R.E. and it is difficult for the inexperienced to imagine the tedium of the sappers’ unromantic and apparently losing struggle against the persistent water which eroded the defence and even gnawed away at the soldier’s morale.
Whether you would consider this biological warfare or not is debatable.
It is unclear from his memoirs what period of the war this referred to but the immediately following paragraph refers to the death of an
Sgt. V. Caudle NCO i/c Section 3 who is recorded as dying
My grandfather served in the Royal Engineers in WW1 (and WW2 for that matter). At the above date he was serving with the 28th division.