I think this patch gives poison positive weight when attacking a unit that will advance. Poison damage should probably be ignored in a kill-or-advance situation.

This patch also appears to give poison its full weight when attacking a unit that will be left with very low health; generally such units don't suffer much from being poisoned (it just makes healing them take 1 turn longer). Not sure how easy that would be to fix, though.

Consider the fact to poison a unit who will survive after the attack.

(file #15740)

Third version:

I only add 1 to the rating value if the unit is not poison AND the attacker can poison him
Second, I take into account the chance of an unit to be poison if he is not.

(file #15717)

Units must be capable to spread poison around.

I add a boolean "is_poisoned" which is true if the unit is poisoned at the beginning of the battle.
If he is not, I had +1 to the rating value.