What happens if you have a yeast infection and do not treat it?

Depends on details: Yeast is a life form, and there are many kinds. Some live with humans, as a normal part of us. If these "friends" of ours get to be more than desired, we call it a yeast overgrowth... such as when seen in the mouth, on the skin, in the vagina, etc... If they get even more overgrown, or if any yeast go where they are not supposed to live, we call them infections... which can worsen if untreated.