What happens if foot fungus goes untreated?

Which foot fungus?: Depends on the foot fungus. Athlete's foot can persist and can spread. Nail fungus can spread to other nails and become quite persistent. If there are other medical conditions they can complicate the infection, and they can be dangerous in immunocompromised patients.

Sometimes nothing: Other times it can spread...Can spread to different body parts. If it is inbetween toes it can cause a deeper infection thru cracks in the skin. Oftentimes, fungus on the skin will cause fungus on toenails.....Best to get rid of as usually a cream for a couple of weeks is pretty effective.