In act 4, scene 3, Malcolm tests Macduff's loyalty before admitting that he would make a proper king and elaborating on his plans to attain his rightful throne. While Macduff attempts to collect his thoughts and process everything that Malcolm has told him, a doctor enters the scene and mentions that a crowd of sick people are surrounding King Edward, who is miraculously healing them with a simple touch. The doctor also mentions that King Edward's healing touch is a gift from heaven and Macduff asks what disease the king is healing. Malcolm replies by saying, "'Tis called the evil" (Shakespeare, 4.3.148). This evil that Malcolm refers to is also known as "the king's evil," which is actually a disease called scrofula. Scrofula is a form of tuberculosis infection that occurs outside the lungs, which causes the inflammation and irritation of lymph nodes in the neck, making them swell up.

In the Middle Ages, it was believed in England and France that "the king's evil" could be healed by being touched from royalty. In the play Macbeth, King Edward is said to have this miraculous gift sent from heaven. King James I was also said to have the royal touch that would heal the disease known as "the king's evil."