蛇

How it works

This is a passive sentence — the snake is what acts, but the focus is on being on the receiving end. に (by) marks the agent doing the biting (蛇(へび) (snake)), and 噛む(かむ) (bite) shifts into its passive past 噛まれた(かまれた) (was bitten). The person who got bitten is left unsaid — context carries it.