Actually, guys, all of your advice, though gold it is, proved to be unnecessary.
I thought of something like that, maybe, giving them tough decisions to make, making them harvest local wildlife/water wells or something, with an idea to improvise
upon getting to the village they suddenly became very suspicious of those that weren't afflicted (as if a normal Endurance check from their side is impossible).
In short, I went with that, and in the very end they managed to find proof of inner conspiracy (Vecnian symbols), although most of it was a bitter fail, since they lost like pretty much everyone, except for this one guy they saved that is actually one of the Vecna cultists, and one other woman who fled, which was a cultist as well.
Just out of curiosity, how did these people contract said diseases in the first place?
Well, it's a long campaign plot, but - a new "Cult of the Liberated" (Tharizdun's, he's free, nvm) suddenly appears in a small, secluded valley and starts it's conquest. In order to protect one small town from the fanatics, a guy named Mean who is a captain of the Legion (nvm) makes a deal out of desperation (and imprudence) with some old one-eyed man. Undead start spawning in the loess valley around the town, preventing the attacks. Soon, the town itself is engulfed in necrotic and Mean becomes a Death Knight under Vecna. The Undead became so successful that they pushed back the Cultists and took control of almost the entire valley.
So, original plan was that, since undead appear for no reason, some of the few villagers that survived in that remote place got the disease from eating carcasses or from dead animals and graves; you catch the drift. Afterwards, when the PCs were heavy on "that grandma is really suspicious, I should just tie her down or kill her!", then I decided on intentional sickening by the Vecnian necromancers.
Thanks again, guys. Wish this forum had a heart smiley.