Ringworm is caused by a fungus called a dermatophyte, not a worm. Fungi grow best in warm, moist environments, such as locker rooms, showers and swimming pools. In humans, the fungus lives and spreads on the top layer of the skin, hair or nails. Ringworm is contagious and can be spread by skin-to-skin contact with an infected individual, or by sharing contaminated clothing, towels, combs, hats or bedsheets with someone who has ringworm. The fungal infection can also be picked up by touching an infected dog or cat.