Ringworm is not caused by a worm. It is caused by a fungus. The kinds of fungi (plural of fungus) that cause ringworm live and spread on the top layer of the skin and on the hair. They grow best in warm, moist areas, such as locker rooms and swimming pools, and in skin folds. Ringworm is contagious.It spreads when you have skin-to-skin contact with a person or animal that has it. It can also spread when you share things like towels, clothing, or sports gear. You can also get ringworm by touching an infected dog or cat, although this form of ringworm is not common.ingworm is contagious. It spreads when you have skin-to-skin contact with a person or animal that has it. It can also spread when you share things like towels, clothing, or sports gear.