Platelet

Platelet is a rogue-lite action-platformer for the NES. You play as a microscopic germ on a mission to infect a human body. The game is built around a core loop of repeated runs through an evolving set of levels. Each attempt triggers a new "immunoresponse," where the body's immune system (the level itself and its enemies) adapts and grows stronger, forcing the player to rely on a rogue-like upgrade system to progress.