Athlete's foot is caused by a fungus that grows on or in the top layer of skin. Fungi (plural of fungus) grow best in warm, wet places, such as the area between the toes.
Most of the time, a doctor can tell that you have athlete's foot by looking at your feet. He or she will also ask about your symptoms and any past fungal infections you may have had. If your athlete's foot looks unusual, or if treatment did not help you before, your doctor may take a skin or nail sample to test for fungi. Read more about athlete's foot.