How did roger chillingworth become Dimmesdales medical advisor?

Roger Chillingworth became Dimmesdale's medical advisor when he arrived in the Puritan community as a stranger and recognized Dimmesdale's deteriorating health. Chillingworth leveraged his skills as a physician to offer his services and gain closer access to Dimmesdale due to his genuine concern for his health and his ulterior motive to discover his secret sin.