Healing even a single hit point removes the danger (but leaves a severe debuff in turn that can only be removed by certain campfire skills or finishing the quest), but having a heart attack will automatically bump them down to Death's Door, and a heart attack while on Death's Door is 100% fatal. To somewhat compensate, reaching zero Hit Points doesn't result in instant death or even a knockout the character is still awake and functional, but is at Death's Door, where any subsequent damage may kill them note Death's Door is even considered a status effect, with the ability to buff your resistance to a killing blow, and there's an achievement for resisting death five times in a single battle. If your favorite hero that you've invested lots of money in dies, that's it.
This means the Crusader, Leper, Arbalest, Man-at-Arms and Bounty Hunter never take off their armor.