Fungi are present everywhere in our environment, including on the human body. They thrive in warm, moist areas, such as locker rooms and swimming pools, and in skin folds. You can get ringworm of the skin by sharing contaminated towels, clothing, and sports equipment, and by direct contact with an infected person.
Ringworm of the skin (tinea corporis) is most commonly caused by the fungus Trichophyton rubrum, which spreads from one person to another. It can also be caused by Microsporum canis, which is spread by cats and dogs. This type is less common but causes a more severe infection.
People often get ringworm of the groin ("jock itch") by accidentally spreading athlete's foot fungus to their own groin area. People who have athlete's foot also commonly spread it to their hands (tinea manuum).
Some people are more likely to get fungal infections than others. The tendency to get fungal skin infections or to have them return after treatment seems to run in families.