Mouth breathing increases illness proneness because the nose filters germs better than the mouth.

“This is why we're heading into the winter months being a nasal breather. That sounds like crazy new agey stuff-- be a nasal breather. There's actually a book written by Paul Ehrlich and Sandra Kahn at Stanford with a foreword by Jared Diamond and Robert Sapolsky. The book, Jaws, so these are some heavy hitters. And there's very good evidence that people who mouth breathe are making themselves more prone to illness when nasal breathing because of the microbiome. It's just a better filter for germs.”