I had a probably stupid idea that might actually be educational

Say your people are busy saving the world. Say they've saved it. Doesn't matter. This can apply to the best of players, and happen at any time.

Spread rumors of a new virus. This virus is memetic in nature; it propagates through a pamphlet. When you read the pamphlet, you contract it. If someone explains the information in the pamphlet to you well enough, you contract it. If you ingest some infected person's blood or mucus, you contract it.

Who are the infected? What does this virus do?

The infected are odd creatures. They dress weirdly, alternating between flamboyant poufery or incredibly drab clothes. They have little to no regard for money, hoarding it selfishly but rarely spending it except on new equipment or purported magic items. They love their equipment, but they mistreat and ignore it when not in battle or busy burrowing through the warrens of some intentionally buried necrothing crypt.

They don't really talk to strangers, and move in packs. People avoid them instinctively, flowing around them. They are weird. They are outsiders. When they talk it is stilted and often simple.

They look at each other constantly. When they talk to each other, it feels like round words being forced through square mouths, off-kilter.

To a creature, they are deadly in battle. All their battles, except for spells, are fought in complete silence. They die quietly, eyes flicking furiously around. They take no heed of lethal wounds.

Anyone close enough to them can sense the wrongness. The fundamental disconnect. The jerky, spastic movements they make. They are like puppets, moving in some strange dance, some like a standing corpse, others loosely animated.

When your players come across a pamphlet, it needs to be nasty and grease stained. Trampled underfoot in the gutter. It should be badly printed and unintelligible.

It should be, with no spite intended, a summary of the Player's Handbook.