This is from Act 5, sc. 1 and it is spoken by the doctor who has been called to the Macbeth's castle to tend to Lady Macbeth. He is saying that people who have done a deed that will cause them to have a guilty conscience, will suffer from the guilt that the deed causes, particularly if the "unnatural deed" is the rumored killing of the king. He goes on to say that people suffering from such a guilty conscience are known to walk or talk in their sleep as a way of purging their minds of their guilty secret. The doctor knows that rumor has it that Macbeth may have killed Duncan and others and he realizes that Lady Macbeth, knowing about or having a hand in these deeds, most likely suffers from a very guilty conscience. He says, too, that Lady Macbeth needs religious intervention to help her mind and her soul, not anything he can give to her. She doesn't have a physical problem, she has an emotional or mental problem.
This insightful remark stems from both the doctor's observation of Lady Macbeth's unnatural behavior, what she says, and the remarks made by her gentlewoman. The doctor had earlier commented that Lady Macbeth's sleepwalking was "A great perturbation in nature," which suggests that her behavior was unnatural.
When he enquires what remarks Lady Macbeth had been making in this unusual state, her gentlewoman refuses to divulge any information, even when he insists that she may, to him. This makes it obvious that the lady had been mentioning incriminating details about her involvement in crime whilst walking in her sleep. The gentlewoman is, of course, afraid of betraying her lady by mentioning details of what she had heard her saying about her complicity in others' deaths.
When the doctor actually hears what Lady Macbeth says, he is quite intrigued. He remarks, "Well, well, well," which indicates that he has just been enlightened about something he had not been aware of. Lady Macbeth's whispering and actions are, in fact, a confession of what she has done. She continuously rubs her hands, attempting to remove an unseen stain and stating that it is a permanent blemish that can never be washed off. She utters that there is blood on her hands. The doctor muses that her "heart is sorely charged" which means that she is bearing a heavy burden and that she is deeply disturbed.
He had already declared that he does not have the means to cure her of what he suspects is her overwhelming guilt.
This disease is beyond my practise....
His statement that he knows of those who had walked in their sleep but died "holily in their beds" suggests that they had found redemption, which he believes Lady Macbeth is seeking.
Lady Macbeth's direct reference to Banquo and her contention that what has been done cannot be undone convinces the doctor that she is, unknowingly, confessing her guilt for having been part of terrible crimes.
It is then that he mentions that there is the suggestion of something foul permeating the country and that any act which goes against nature would result in unnatural outcomes and bring unwanted complications. In this instance he means that any crime becomes a breeding place for greater perturbation and problems.
The doctor is suggesting that Lady Macbeth's thoughts and her conscience have been diseased by the unnatural deeds that she has been involved in, thus causing her great harm. Her conscience is eating away at her and those like her will ordinarily confess their misdeeds to things which cannot hear, to rid themselves of guilt. In such a situation, their confessions would be inconsequential.
Lady Macbeth, however, has been completely engulfed by remorse. He suggests that she needs divine intervention for a cure because he cannot heal her. In other words, Lady Macbeth must seek forgiveness from God; human intervention will not save her.