That both happens in the same episode. Kenny gets the chickenpox, and Cartman's, Kyle's, and Stan's parents all want them to get the chickenpox while they're young so it doesn't get worse (the younger you are when you get chickenpox, the less harmful it is.) And when going to Kenny's house for a sleepover so they can catch the chickenpox, Cartman sings "In the Ghetto" referring to Kenny's poor house. His family is notoriously lower-class.
