Athlete's foot and toenail fungus are caused by the same type of fungus: dermatophytes. While both are contagious, it is more likely for a toenail fungus to be caused by Athlete's foot, a fungal infection of the skin, than the other way around.

While toenail fungus is contagious, it is encapsulated beneath the nail. In essence, the toenail protects the surrounding tissue from the infection spreading. The opposite is not true. A fungal infection of the skin, Athlete's foot, can easily spread to the toenails. For this reason, it is vital to treat Athlete's foot until it is completely resolved