If you read well the replies other gamers posted almost the same i said.
Fair enough and I apologize for the tone of my reply. The problem is that what you and other posters are suggesting - that followers cannot be killed, except at the hand of the player - is known to be false. Not a lie, and no offense is meant. Just commonly-held, but incorrect information. I went about 33 levels in the game without ever seeing a follower killed except by my own errant swings or spells,so I know where you and others are coming from. Lydia appeared invincible and the guide book incorrectly makes the same claim, so there is ample reason to believe that followers can't be killed except by the player.
The thing is, it only takes a single example to disprove the claim, and we have many more than a single example. Nevermind my own experience:
I've had Lydia die
when a dragon priest kept hitting us with an AoE fire spell. I'm sure i didn't kill her myself as I was too busy running away to save my own hide
when I was fighting against the Snow Troll and it could 2 hit me... my NPC got knocked out over and over. Eventually I found them dead
as I was running around.
Previously I quoted from the "discussion" page of the Wiki, but here is the main page description of the follower death mechanic:
The game treats permanent (non-canon) followers in a similar way to "essential" NPCs, but still allowing the possibility of their death. When your follower would die to an enemy attack or a trap, they will be incapacitated for some time and begin to recover their health, just like essential NPCs; enemies will stop attacking or targeting them during this time. However, if they lose all their health again during this period, they will die for good. Thus, you should try to move the fight away from them, so that they are not hit by a stray arrow or area-effect spell. They can also die if hit with a damage-over-time poison that is stronger than their health recovery. They can also die from enemy power-attacks.
I guess just be careful out there; you don't want your pack mule/decoy dying at the wrong moment!