What does it mean if a boy says I'll chase you after you get the pox?

Maybe 1: You like him, he knows, but he doesn't like you. When he said this, he was joking and refusing you.

Possibility 2: He likes you a little, but he thinks he can't stand some of your defects (acne) now.

Possibility 3: Just kidding, maybe he will forget when you get the pox.