Kerion:
A parent's guide to condition and treatment information

A kerion is a scalp condition that occurs in severe cases of scalp ringworm (tinea capitis). A kerion appears as an inflamed, thickened, pus-filled area, and it is sometimes accompanied by a fever.

The underlying condition, scalp ringworm, is a usually harmless fungal infection of the scalp and hair that occurs as scaly spots and patches of broken hair on the head. Ringworm of the scalp is most commonly seen in children. Though several different species of fungus may cause scalp ringworm, they are generally known as dermatophytes. Scalp ringworm may be acquired by direct contact with infected people or with contaminated objects that have been handled by infected people (such as combs, pillows, and sofas). Most commonly, scalp ringworm infections are caused by dermatophytes that prefer to grow on humans. Less commonly, the fungus may be spread from infected animals (zoophilic dermatophytes) or from the soil (geophilic dermatophytes).
 
Kerions usually occur in people who have been infected with zoophilic dermatophytes. A kerion is believed to be an overly active response of the immune system or an allergic reaction to the fungus.