Passive Heal is a free passive ability that all Chaos Units have by default. It cannot be toggled off.
Passive Heal is fairly self-explanatory. All Chaos Units will passively regain health at a slow rate. This healing occurs at all times, even in combat and while moving.
Passive Heal also automatically cures any Chaos Units of Poison after several seconds. However, it is still possible for a Chaos unit with low health to die from poison before it is automatically cured, as the poison can do more damage than Passive Heal regenerates.