In Shakespeare's Macbeth, illness and medicine are tied directly to Lady Macbeth. There is also an illness of character with Macbeth. For both people, they lose the power they had enjoyed previously.
In Act V, Lady Macbeth is sleepwalking and talking in her sleep about the murders she and her husband have committed or been involved in. Whereas she was so strong at the beginning, now she is losing her mind.
Macbeth calls for the Doctor, and the physician and Gentlewoman observe her. They hear things they would rather have no knowledge of. Macbeth asks for an update:
How does your patient, doctor? (42)
The doctor responds that she seems to have a troubled mind. Macbeth responds by asking him to cure it:
Cure her of that.
Canst thou not minister to a mind diseased,
Pluck from the memory a rooted sorrow,
Raze out the written troubles of the brain,
And with some sweet oblivious antidote
Cleanse the stuff'd bosom of that perilous stuff
Which weighs upon the heart? (46-52)
When the Macbeths were loyal to King Duncan, they were well, as was the country. As soon a they murder Duncan, unnatural things begin to occur because the universe's balance had been upset with the unnatural death of Duncan. When they start on this downward spiral to murder and deceit, both are robbed of their power. Lady Macbeth, who had been so strongly committed to their plans and a powerful woman in her own right, has been diminished to a worried and fragile ghost of the person she once way, haunted by their actions.
In Macbeth's case, his mind seems unwell as he envisions the daggers leading him to Duncan's chamber, he turns to quivering jelly when the deed is done and he returns with the bloody dagger; his men believe him to be mad when he sees Banquo's ghost; and, ultimately, he becomes a murderous butcher, which is not the person Macbeth was. The man once known as a fierce warrior and fearless supporter of the King, is now losing the respect of his followers who smile with their faces but not with their hearts; they do not trust him, and once Malcolm and the English army move toward Macbeth, he has no power to command his men who are starting to change sides to fight with the English. As this comes to Macbeth, he states, "I am sick of heart..." (V.iii.20) as his sense of power dwindles.
The illness or disease of Macbeth's soul and wife's soul and psyche leads to the loss of personal power—poisoning the mind—which in turn, makes them both powerless.