Family Guy: Season 15, Episode 6 - ProjectFreeTV

15 - 6: Hot Shots

In order to "save" Stewie from autism, Peter and Lois decide not to vaccinate him and then proceed to convince the rest of Quahog to follow suit. Stewie runs away to save himself from diseases until Sean Penn (guest-voicing as himself) swoops in to save the day and brings vaccines to the whole town.