Elephants crossing the seas!
It is not an infectious disease, but it is a plague upon the world.
For it spawns villains. And not your garden variety villains, but epic villains, master villains of the most dangerous kind Evil Geniuses.
Pnathic sorcery has existed for ages, crudely used by magi who no idea the true forces they were fumbling with.
An article concerning the nature and role of druids in an RPG enviroment.
Plagues and other great diseases are important parts of history and some settings. They defy man and may even defy God in their swift and terrible punishment upon the body of society.
The players see a small shrine to the local nature deity just of the trail. Before they even approach it, they can smell the foul stench of rotting meat. If they inspect the shrine, they can see it has been desecrated by rotting organs in the last few days. There is no mistaking it for an obscure ritual, the organs are thrown everywhere, not left in specific places as in sacrifice.
If the players try to clean the shrine, they will soon find it has been boobytrapped to fling sharp splinters covered in the rotting gore in every direction. While only doing a few points of damage, they injured players will likely take sick soon unless they get medical attention.