The failing health that greatly affects Reverend Arthur Dimmesdale is described to have begun around the same time that Hester's ignonimity began. This, of course, is because the guilt and shame that ate away at his ego were causing his entire esteem to disappear. As a result, he is a weakened, almost broken man. Chillingworth is aware of this almost securely, going as far as suggesting to Dimmesdale that his illnesses are caused by otherwordly causes, mainly set in the heart. Chillingworth wants to probe Dimmesdale, and hints at sin as the potential cause of his maladies.
A bodily disease, which we look upon as whole and entire within itself, may, after all, be but a symptom of some ailment in the spiritual part. Your pardon, once again, good Sir, if my speech give the shadow of offence. You, Sir, of all men whom I have known, are he whose body is the closest conjoined, and imbued, and identified, so to speak, with the spirit whereof it is the instrument.
We know, as readers, that Dimmesdale himself is responsible for his own failing health. A man once seen by his fellow villagers as a "young divine", his Oxford education, his good looks, slovenly demeanor and soothing manners made him into a pillar of the community; a man of superstar qualities who epitomized the very essence of decency, glamour, and fascination. Dimmesdale's main flaw of character is his insistence in living up to the image that the people have made of him. He wants to be the young divine. He enjoys being looked up as an elder, even being so young. He imbibes the pleasure of feeling loved and admired; perhaps those very factors were pivotal in the development of his relationship with Hester.
Now, however, the romance is gone. Hester became pregnant, which automatically makes it obvious that he is none of the things that he has come to believe himself to be. His lack of courage to face up to what he did makes him a coward. His inability to come in Hester's defense as fiercely as he should makes him a weak man. The fact that he allows Hester to suffer the shame of his sin on her own makes him selfish. The lack of interest in being true to the promises that he makes Hester makes him pretentious. The fact that he continues to adhere to the "character" of "the young divine" of the village, makes him look even more preposterous to the witnessing eyes. All of those flaws of character are eating away his ego. It is his ego, and nothing but that, which keeps Dimmesdale ill to the point of near death. He does have problems of the heart and the soul.However, he also has the biggest of all problems which is the unwillingness to let go of the lie that he has created for Hester, for himself, and for everybody else who ever believed in him.