Most reasonable way to handle it (in my mind, at least--which is not always reasonable), would be to have assassins "poisoning" towns by "destroying" baths, sewers, etc. I don't really use assassins for sabotage, so I don't even know if destroying health buildings increases squalor. If it did, that would be the best way to imagine poisoning tactics, if you should so desire.
Might also make another interesting logistics trait..."poisoned water supplies...-x% movement, -y morale"
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