If four out of five people suffer from diarrhea...

If four out of five people suffer from diarrhea does that mean that the other one enjoys it?

This is bathroom humor. Talking about diarrhea is generally considered impolite, so the shock value of this question is part of the humor. The rest of the humor comes from the incorrect assumption that everyone has diarrhea. A more likely parsing of the sentence would mean that the other person does not have diarrhea, not that the doesn't suffer from it.