What excuse do Macbeth and lady Macbeth give for his behavior at the banquet?

Lady Macbeth tells their guests that this behavior is something Macbeth has been suffering from his entire life, and that they should ignore him.

Macbeth has suddenly gone mad during the banquet because he saw the ghost of Banquo, his former friend whom he murdered due to his ambition. He starts rambling about seeing the ghost (when no one else can see him), so everyone is startled until Lady Macbeth tells them to ignore him.