In the play, Angels in America, what was Prior Walter's first indication he had AIDS?

After Prior accompanies Louis to his grandmother's funeral, both sit on a bench.  At this time, Prior discloses to Louis that he has a leision on his arm, what he calls the "wine dark kiss of the angel of death."  This mark is Kaposi's sarcoma, an indication of the onset of AIDS.  At the time, a mark like this was literally seen as an infection akin to the plague, and this is the first indication that Prior has been infected with the virus that causes AIDS.  At this point in the play, we only now grasp that Prior has had it for some time, and his concealment from Louis is from the fear that he will leave.  As it turns out, it is quite a valid concern, as Louis reacts angrily to the concealment, but might be masking the theme of abandonment which will be developed more in the characters of Louis and Joe.