To what disease is Macbeth referring? How is this ironic?

In Act 5, scene 3, Macbeth gets a report about his wife's apparent illness from her physician. The doctor says that she is "troubled with thick-coming fancies / That keep her from her rest" (5.3.47-48). There is nothing physically wrong with her; her illness is in her mind. Macbeth's response is somewhat insensitive. He says,

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 stuffed bosom of that perilous stuff
Which weighs upon the heart?  (5.2.49-55)

In other words, Macbeth expects a doctor of the body to do something to Lady Macbeth's body in order to change something happening in her mind. He wants the physician to somehow "minister" to his wife, to somehow extract the terrible memory she keeps reliving, to destroy her mental troubles, or perhaps give her something that will make her forget whatever it is that so weighs on her. His expectations are ironic given the nature of Lady Macbeth's ailment: she feels guilty, and there is no medical cure for guilt.

Macbeth's response is ironic, as well, because he sounds as callous as his wife once did in regard to guilt. Immediately after the murder, she scolded him for dwelling on his guilt, saying, "These deeds must not be thought / After these ways; so, it will make us mad" (2.2.45-46). Now she has actually been driven mad as a result of her guilt, and her once-weak husband is the one who acts as though it is so easy to simply put one's guilt behind them. We would likely not expect such a reversal in attitude.