Ah, if it's a disease attribute (or a mutation), then it's possible to see it under your feat's (and you get an additional feat every five levels, and more skill points every level)
Cure Corruption for Disease, Cure Mutation for Mutation.
But, you only have to be exposed momentarily for the ether disease to start progressing again after curing yourself, as far as i'm aware, but, as you do more quests and more dungeon runs, you'll tend to be able to stockpile more potions in your home, or another such safe place, than you use.
The easiest Cure Corruption potion you get though, is by heading down to L3 of the dungeon south of your home, talking to the scout, then talking to the king in Palmia.
As for the guards attacking you with positive Karma, that is odd, and i can honestly think of no reason why they should, as i've murdered citizens infront of guards before for merely stepping in my way over and over again, or standing in a way that blocks me leaving, without them attacking me.
On that note, whenever I go to Yowyn, i kill Gwen the Innocent, as i get no Karma hit for it, and the first time you do so, you get to steal her arms and armour, that netted me this time, a longsword of lightning, that i've long since given to my pet. Of course, killing Gwen requires you're strong enough to initially, though anyone with Magic Arrow can do so in a few hits, easily enough.