Poisonous
Poison makes the affected stack lose 20% of its maximum Health each round for 3 rounds (cumulative). In other words, the unit's health is reduced 20% in the first round, 40% in the second round, and 60% in the third round. Poison doesn't do damage to any of the units, but by reducing the stack's maximum health, it makes each unit easier to kill.
For example, a stack of Pikemen would have 8 hit points per unit during the first round affected, 6 hit points per unit during the second round, and 4 hit points per unit during the third and subsequent rounds.
The only way to remove poison before combat ends is the Cure spell.