Zombification disease?Really anything works, as long as it spreads faster than the number of spell slots available for Cure Disease allows for curing.
I have a partly finished Poison/Disease cleric domain for Talona done, can post if desired. Talona's clergy spends as much time curing disease as they might spreading it... or rather, the good ones heal, the bad ones spread, and some do some of both depending on who needs to die.
Neverwinter Nights had a disease as a major plot point.
1. Establishing the disease as a threat.
Side effects that create things for PCs to do include: Quarantines disrupting trade and making people unsafe. Banditry. Bodies piling up -> undead. Corruption, smuggling.
2. Researching the disease.
This could be the PCs, or it could be NPCs they help.
Quests/things to do: Gather special tools, get samples from different areas, solve a burglary at the research area, retrieve and escort a subject matter expert from bandits/kidnapping/evil Duke.
3. Curing the disease.
Retrieve special ingredients (in NWN, they were an Umber Hulk, a Nymph, a Rust Monster, and an Intellect Devourer), protect a magical ritual site, bribe/steal/etc. and transport 3,000 lbs of Nerrhavian Snow Salt, etc., prevent sabotage by the enemy.
4. The creature behind the curtain.
This was all a diversion to weaken XYZ in favor of a plot. Kingdom takeover, return of ancient lizard people, cultists of Extraplanar Evil, Aboleth, dragons playing a long game, one deity trying to weaken the faith in another deity, or whatever you want to fight.
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