Some critters (Dragons surely) do not permit mercy or unlimited chances to recover whereas, generic mobs will provide uncounted chances to recover. If left alone long enough or healed by you they never die. When they run out of health they kneel asking for mercy and/or a chance to recover. Of course you could also use the "heal other" spell thats available, and prevent them from getting disabled, but wheres the fun in that?! Oct. Lydia kept getting in my way, lol! If you do pull the aggro off of your follower and finish off all the baddies, they should return to full health. Just like everyone else has said, if you directly (or accidentally) attack your follower, PAST THE POINT OF THEIR "DOWNED" CONDITION, you will kill them. It regens health super fast as I watched it tank a dragon without issue. I felt bad at first, but he'd been glitched for a couple missions so I was done with him anyways =) The horse you get from doing the Dark Brotherhood quests (Shadowmere) is nearly indestructible though. I accidentally nuked my follower with a destruction fireball spell. Thankfully, I already tapped him for all the archery training he could give me. My follower Faendal was killed by a Spriggan while I went to retrieve sap for The Blessings of Nature quest. 10 answers to this The Elder Scrolls V: Skyrim question