The poltergeist follower is one of those bad ones, clinging onto you and forcing you to only move 1 space per turn. It says you can only rid yourself of it by crossing the River, either by bridge, raft, or ferry. You must then discard it.
The last time I played this, a character got this, and we interpreted crossing the river via bridge to also include going from the City to the Fields (and vice versa) that takes you over the river (just look at he game art, there's a bridge and everything). Also the same applies to the Village on the opposite end of the board.
Are we in the right? was it intended to include these bridges, and not just the sentinel?