Maybe this was my misunderstanding; but, I thought companions could not die in battle? Lydia and I had become separated, and when I finally was able to get in the room she was in, she no longer roamed Skyrim. Easy enough fix....just pick a prior save; but, i was surprised. i had thought they just got knocked out until the battle was over. Does this mean any follower can die in battle? If so, that changes my tank mentality.

Apparently so.......

Some followers (7, I think) are marked "essential". Meaning they cannot be killed. However, others are "protected" and will go down on one knee but can still be killed if you hit them yourself.

Here's the skinny on it... http://www.uesp.net/wiki/Skyrim:Followers#Follower_Death

You can set followers you don't want to die to essential via. Alternatively you can teleport your followers dead bodies to you and resurrect them, but they will lose any items you give them. I do not recommend teleporting to your dead followers as you end up in the after life with a daedric prince and a pile of bodies. Id show you an image but I cant post links.

Yeah.. that was more than a little creepy when I did that. Was looking for a shopkeeper who apparently died in a vampire attack or something.

Be careful with spells like fireball and chain lightning when using a nonessential follower. If your follower is kneeling, and you blast them on accident, they're history.

I used to have non essential followers (Faendal, Janessa) die on me all the time so I got me an essential follower (one who can't die, I have Mjoll right now). They are not easy to get coz I had to go through a quest in a Dwemer ruin. Anyways, if you want to have non-essential followers, give them a bunch of healing potions so they can recover their health during battles and give them good armor so they won't die easily. Good luck!

Technically, yes, they can die, but it's unlikely for them to because they usually just fall to one knee and the enemies will ignore your followe until he/she gets up.

Essential followers can't be killed by anyone but the player.