Ringworm is a fungal infection that affects the skin. It spreads from person-to-person. Ringworm appears as a round or oval patch. It is smooth in the center with a scaly, red border. The most commonly affected areas are the scalp, feet, nails, and groin. It's called ringworm because of the way it looks. It's not caused by a worm. Ringworm is not serious and can usually be treated at home.