Ringworm is caused by

Ringworm is caused by Fungi.
Ringworm is a common infection of the skin and nails that are caused by fungus. The infection is called “ringworm” because it can cause an itchy, red, circular rash.
Ringworm is also called “tinea” or “dermatophytosis”. The different types of ringworm are usually named for the location of the infection on the body.