By chance a dragon-slaying quest took me near one of the few evil lands on the continent, a mountain range called the Infected Crest. I was advised to go see a law-giver on the other side of it, so it wasn't too far a detour to try to scale the central peak. I didn't see any evil creatures, any undead, or anything that wasn't typical of semi-savage mountains, except for a few clouds of 'haunted vapor'. A few animals got coughing fits inside, but after it passed, I didn't see any changes. I ran through it the thick of it myself, but it didn't do anything aside from the coughing, which subsided after I left it. I've had a few days of resting since then, and haven't seen anything wrong. Maybe it's one of the weaker syndromes, and maybe High Disease Resistance prevented infection.
I carried on to see the law-giver, in a keep isolated from the rest of civilization. He turned out to be a demon -- I'm not posting a save, but there's a composite screenshot:
He's posing as Lomoth, the god of law, discipline and suicide, but notice I addressed him by his real name 'Cadedi'. He wrote scores of books pretty much giving away the secret, with titles like 'I'm a lying skunk devil'. They were scattered all over the keep, and I read a few before meeting him. He didn't seem to be bothered by it; maybe he'd been hoping to meet a human who wasn't such a complete idiot