What bad news does Prior deliver to Louis the day of Sarah Ironson's funeral?

In Act 1, Scene 5 Louis and Prior sit on a bench outside the funeral home. Prior takes the opportunity - while "death" is the subject - to show Louis his first Karposis Sarcoma lesion. This lesion is a definite sign that Prior's AIDS has severely affected his immune system. He waited to tell Louis for fear that Louis would leave him.