2x23 "The One with the Chicken Pox"

Phoebe catches chicken pox just as her old flame Ryan (Charlie Sheen) arrives in town on leave from the Navy. Even though he hasn't yet had chicken pox, he can't stand to be apart and they both spend the rest of the time sick and trying to not scratch at the sores. Joey gets a job at Chandler's office but decides to play it as a role. The character he creates, Joseph, has a wife and kids and ultimately blames Chandler for his own screw ups. Monica is upset that Richard doesn't have a "thing": an unexplainable quirk. He eventually comes up with one to ease her mind.