Actually, followers will die if they go down on one knee and the enemy keeps attacking. You are not the only way they can die.

I don't like the melee companions. All they do is get in the way and with friendly fire on it's way to easy to kill them accidentally. I much prefer to have a caster along if I'm taking a companion. They stay back out of the way doing ranged damage. I can attack any way I like without worring about killing them. The only thing I have to really watch is the shouts. But yea none of the enemies can kill them so you did some damage to Lydia when she was down on one knee and that's why she died. It's pretty stupid they made it that way to be honest. It's especially bad when you play a mage because you can use any aoe spells if you have a melee companion. Not sure why my mage can control his aoe enough not to damage himself but he can't control it enough not to harm friendlies.

One thing you have to learn in this game, there are some fights you have to walk away from and return later. Sounds like this would have been one of those fights.

If you need a follower that badly in a fight, you're in over your head.

Also, if you're on the PC, you can make her (and other followers) essential, and she can't ever die.

Good luck.

PS: You can give followers health potions, and they will use them as necessary.

PPS: Give her a fireball and frost staff. She dual wields them and is overpowering with them.

I've played about two months now, so I'm a little late to this party. And I've had Lydia since that time. I have three other Housecarls now also, but they just hang around the house and guard my stuff (I feel sorry for them, they look so sad). I have added a Mage/priest (that guy from Nightcaller Temple) who is quite the gun with Fireball staffs. I save him for places wtih Necromancers and Vamps.

Anyway, Lydia. Besides being the best to look at, with the best voice, she's not the pushover many here seem to think. As many have stated, you have to be aware that you have a friendly. Much like real life. It makes "life in Skyrim" that much more interesting. Add a dog (Meeko) and you have quite the traveling troupe.

But friendly fire death is always a problem. I've hadn't had any issue with Falmer poison (and when I see them take hits, I pause a moment to allow them to recover their full health before marching on). And better armor/weapons and plenty of Health potions makes the difference.

And yes, Meeko charges ahead and right in my line of fire. But he can take down a Dragr with no help. Melee fighting can be dicy as well. But that's the nature of "my" part in Skyrim. I enjoy the company and Lyd's smart-ass comments. As for her stepping in front, remember, she's sworn to defend "me" with her life. I think that's why she does that, when the Erundal doesn't.

Its all about planning and tactical use of your followers.

If babysitting isn't fun fo you, don't use a follower. I actually did find it kind of fun with one character. Keep them outfitted with the best, and keep them alive. I found it a challenge. Because they are stupid. They pull things you don't need to fight, they get lost. They'll blindly attack and overpowering number of enemies and die. I don't use them for my sneaky-thief type characters. But they can be fun if you think of it as a challenge. Marry Lydia, and you have to keep her alive because she's your wife, and you love her, but she refuses to stay behind because she's your wife and she loves you. Roleplaying thing. Makes it fun. Sometimes. Many other times it's just a pain in the ass, and she just stays in my house while I do all the questing "solo".

If you are on a PC simple go into the consule and click your mouse on the housecarl and type resurrect problem solved

This is not true for me. I am sure Lydia is leveling up with my character. Note just levels but also the associated health and skills. I know because I've checked it with console commands.

Lydia does quite well even at level 40+. And she really can't be killed unless she gets hit by you or something else when she goes into her kneel mode. That could happen if you accidentally (or purposely) hit her, or if someone hits her. No enemy will target her if she is in knee mode, however an enemy can hit her by accident or if they cast some area effect spell at your character and your character is near Lydia.

I don't use followers at all. On my first playthrough I used Lydia for one mission and that was the end of that. They are horrible. ALWAYS getting in the line of fire, terrible pathing in general. The only advantage I can see is the extra carrying capacity. But between the Steed Stone, and having way more gold than I need, even that's not an issue. I find them more of a nuisance than a help, but that's just me.