Today's world is a new world, filled with panic and fear and mistrust.
In an old world (the world of my youth), parents would get kids sick with the chickenpox on purpose so that they wouldn't have to go through the illness as an adult.
This is also true in the world of South Park, and this episode is centered around this idea.
Of course, the kids don't understand the reasoning behind this, and decide to get thier parents back by giving them herpes.
Overall, this is a good episode. There are a lot of good moments throughout, including Cartman and his calamyne lotion, and his closing line: "It looks like we're all going to need a lot more calamyne lotion!"