The Boston settlement lacks skilled physicians, so the Puritans welcome Chillingworth enthusiastically for his apparent knowledge of both traditional medicine and Indian medical remedies.
Chillingworth fakes being a good Puritan. It's a sin to lie, but lying fools the authorities.
Dimmesdale's health worsens and he is seen often with his hand over his heart. Chillingworth treats Dimmesdale and soon the two move in together.
The novel's two worst sinners now live together. Sin feeds sin.
As Dimmesdale's health wanes, the locals notice that Chillingworth's has transformed from a kind, elderly, and somewhat misshapen gentleman into an ugly evil old man. The transformation makes them suspect that Chillingworth's intentions in getting so close to Dimmesdale might not be entirely charitable: they fear he might have been sent by the devil.
Both Chillingworth and Dimmesdale suffer physically for their inner turmoil. In keeping secrets to hide their sins and conform to social pressure, they cause their bodies, their natures, to wither and die.