... elven ruins? If you actually mean the elven ruins and not the rhaloren camp, a level 8 character should probably not be
anywhere near the elven ruins. That's a zone roughly on par with dreadfell and probably shouldn't be touched until you're in your twenties, at minimum. Upper teens at the very least. Actually, I don't think the ruins can generate vaults anyway, soo...
If you
are talking the rhaloren camp and you had opened a vault... I kinda' doubt that mage was level six. Sincerely. Those vaults (vaults in general generate disproportionately leveled stuff! Bill is your warning, ha.) will happily and generally generate 20+ level critters, and the only elves that casts disease are stuff that doesn't generate in the camp naturally (so you'd be looking at stuff that's level 20 or so, minimum. It's an annoying, but known and apparently persistent issue with the camp.). It wouldn't have been level six, 'cause a rhaloren camp vault that's able to generate those guys will simply not generate something that low a level*
16 or 26 would have been more likely, and damage like that is normal at that point, especially for the latter. In any case, you open a vault, you take your risks. Most vaults don't play around, and can and will wreck you. Survival, especially low level, after opening a vault is not guaranteed and may simply be the end of you**. That's the trade off you make for out of depth level loot.
... anyway, if you're looking to survive diseases, a magical wild will get rid of them if you can't out heal/shield/etc. them. There's also a particular artifact (maybe more than one?) that can clear them out with an activation, if you're already wearing it.
Though yeah,
ghoul diseases, specifically, are something different. They've got ridiculous durations. They've always had ridiculous durations, and for whatever ungodly reason DG and co. have decided to leave it like that despite the issue being brought up numerous times. It's one of the things to remember when facing a vault ghoul... or any ghoul, really, especially the ghasts or ghoulkings -- you don't want to let it into melee range unless you can survive a 20 turn disease. Fortunately, ranged diseases have a much shorter duration. It's only and specifically ghouls that get the ridiculous duration ones.
*In the normal course of things, anyway. There's a known and persistent bug with the camp where it'll occasionally to drop a 20+ level caster on you outside the vaults. Rare, but it happens. Stay on your toes in there.
**Especially if you don't have a ready means to get out of dodge. Opening vaults without a reliable escape method is the russian roulette of T4. Along with farportals, anyway.