Ringworm is a fungal infection that affects the superficial layers of the skin, hair and nails. Despite the name, ringworm is not actually caused by a worm, rather a dermatophyte fungus. The most common fungus responsible in pets is Microsporum canis, but there are many species that can be involved. Ringworm is most commonly spread through direct contact with infected animals. It can also be spread indirectly from the environment, as fungal spores shed from an animals hair or skin cells can survive for months in the environment.