Sounds like a typical "final fever attack", which I believe can potentially kill any low level PC (and possibly high level PCs, but I'm not certain on that).

Final shudder deals 6d6 damage, IIRC. For a mist elf with half HP... yes, you need to be fairly high level to survive.

6d6 is average of 21 hp. So you would have to have ~43 hp to survive it when you are sick, which is even higher when you count the TO penalty for sickness.

Yes, it was a final shudder thing - but wow, it was brutal. Hence, posting under [Balance].
But I suppose there is a reason there are not many Mist Elves. Such a slow rate of evolution means they can't keep up with the latest viruses.
(and they can't be 1-shot killed by Ghost King, so may be a fair trade. Maybe.)

I think this isn't too unbalanced. An exploding door kills you just as much and is much harder to 'counter'. Sickness can be treated in a lot of ways to prevent a final attack from happening (which is harder for some doors).

It would kinda make sense for mist elves to be immune to sickness, given their ephermal nature. Still, there are ways of dealing with sickness (prayer, healer, healing potions) and this is an issue already suffered by low HP characters, so I don't think it's too big a deal.

If mist elves die so easily to everything under the sun, how do they grow upwards of several ten thousand years old? You take control of a nigh-immortal being who nevertheless has only obtained the most basic skills of his or her chosen profession, and you die almost instantly several steps into your first dungeon from being coughed at, or accidentally equipping iron (some of which you've schlepped all the way over the pass into the chain in your backpack, despite your inability to ever use it). Did they live out their days since creation in hypoallergic tents?

You've got a race with very low HP and very slow leveling (which leads to having very low HP even longer) - I don't think there's any reasonable amount of balancing that can be done to change the fact that if you pick mist elf as a race, there is no room for mistakes and you'll probably die from a trap at some point anyway. We can give them immunity to sickness, sure. We can also make traps less deadly, but that means that unless traps do different damage to different races, non-mist elves will barely get tickled by traps past the first few levels. No other race had to have special defensive bonuses just to be able to survive. Gray elves were frail, but at least they followed the same rules as other races and could still stay sort of alive.

I have increased mist elf resistance to the final fever wave as it really does not seem my image of an almost eternal race to be so disease-prone.