Disease

Disease is (per Galen and Aristotle) the failure of a bodily function to function. As organs fail, the victim gets progressively sicker until finally something important gives way, like the heart and - finally - brain.

Traditionally Dungeons & Dragons has likened this to poison sharing the same saving-throw mechanic. Poison works, exactly, to cause organ-failure. More commonly in RPGs disease is associated with transmissible parasites: virus, bacteria, Rot Grubs, you in your mom's belly... anyway the classic low-level example is the sepsis a rogue might get from a rusted lock, or the plague any party-member might get from a rat.

Disease is often used by DMs as a story-device. As the symptoms progress, getting more and more dire, the party knows they have a time-limit, to save their friend. It certainly beats that stupid "ERRK POISON... uhhh :^x" instadeath when the rogue fails to disable a trap.

There exists intersection between the progressive curse and the magical disease. These tend not to be curable by remove curse or cure disease alone. Some monster-descriptions offer recipes on how to fix what they've caused, offering additional adventure-ideas on how to figure out what the DM knows.

One classic D&D example is the Mummy Rot. If you contracted this, your wounds wouldn't heal and you'd decompose. Stronger strains would mean you can't be resurrected by Raise Dead on account, hey, that mummy's gods (usually Egyptian or Inca) have intervened to punish you desecrating their servant's rest.

3e and Pathfinder introduced Ghoul Fever. This follows the Night of the Living Dead plan where you steadily get more and more like a ghoul until you became a ghoul yourself. Meanwhile you could transmit this one to other people.