The Jungian reading
The snake is instinct at its most archaic — the layer of the psyche furthest from the ego, meeting you on its own terms. It carries transformation (the shed skin), healing (Asclepius), and threat at once; which face it shows depends on your current relationship to your own instinctual life. A snake blocking the path marks an instinct that must be dealt with before the way continues. Being bitten, in Jungian reading, is often initiation as much as attack: venom and medicine share a root.