The Old Diseased Man was a man that Roland came across at the beginning of the Mohaine Desert while chasing The Man In Black. The man has long, graying hair (that is receding), boils covering his face and body and appears to be blind in one eye.
In The Journey Begins, he gives Roland a compass, which is his only remaining possession. He believes that since Roland is a Gunslinger and an agent of The White, that the world has salvation, the wheel of life turning. To this Roland tells him "You are mistaken for a colourless world. The problem with turning wheels, people tend to get run over." Roland accepts the compass anyways and The Old Diseased Man begs him to give it to The Man Jesus, should Roland see him.