Some serve useful purposes, or cause no problems. Others can proliferate as infectious colonies. Athlete's foot, jock itch and ringworm of the skin or scalp are caused by mold-like fungi called dermatophytes. These fungi live on dead tissues of your hair, nails, and the outer layer of your skin. Your susceptibility to infection by them is increased by poor hygiene, continually moist skin, and minor skin or nail injuries.
Ringworm often affects children.
The characteristic rings may be irregular and can expand beyond the scalp. The ring grows outward as the infection spreads and the central area becomes less actively infected. This type of infection is very contagious and can be passed from shared hats, combs or brushes and barber tools. It is also possible to be infected with scalp or skin ringworm from pets or domestic animals. Skin changes associated with fungal infections often are quite specific.