I think what If there was chemistry soldier in the Middle Ages.
on the assumption that they are existed... I think they could not use the germ.
probably, they used poisonous gas or black and red pepper.
if so,I think they are practical magician in the middle ages.
they don't expose their skin, were the antigas helmet on their head and a reserve gas bag on their back.
also they carry antidote on their belly and carry the dagger on their calf. dagger was last resort.