That is an interesting question! Shakespeare actually uses the idea of disease throughout the play as a metaphor for the corruption that has crept into Scotland in the form of Macbeth and Lady Macbeth on the throne.
Lady Macbeth talks of life as a "fitful fever" in Act III, scene ii:
Duncan is in his grave;
After life's fitful fever he sleeps well; (III.ii)
She also tells Macbeth he is "infirm of purpose" in Act II, scene ii, indicating that he is sick with regards to being afraid to follow through on their plans.
Infirm of purpose!
Give me the daggers: the sleeping and the dead
Are but as pictures: (II.ii)
And in Act V, scene iii, Macbeth has a conversation with the doctor concerning whether or not a memory that is torturing someone can be plucked from their mind, thus curing the patient. The doctor replies, however, that the "the patient/Must minister to himself" - he (or she) has to repent to cure himself as that is not something a doctor can treat the way he would the body.