Followers do not die like normal NPCs. When they damage enough that would normally kill them, followers drop to one knee to begin recovery. During this recovery period enemies will ignore them completely. However, if they take damage while in this recovery phase due to a stray arrow or spell, they will permanently die.
-If you are the one to damage them for that final hit, they will skip the recovery phase and die permanently.
-While enemies ignore them, it is still very possible for followers to be hit during recovery. It is advised that while follower are recovering, you do not stand near them. It is quite easy for an enemy's attack to miss you and hit (and kill) your follower.
http://www.uesp.net/wiki/Skyrim:Followers
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