What can yeast infections lead to that is bad?

Misery.: An untreated yeast infection can spread on the skin and cause constant itching and burning. The more important question is what is going on in your body that is feeding the yeast. A high sugar diet (this also includes simple carbs like bread, processed flour, rice, corn, pasta) can make it hard to get rid of the yeast. The most concerning possibiliy is diabetes. You should get tested.