Ringworm is a common skin infection caused, not by a “worm” but by related types of fungus:

Ringworm is transmitted by person-to person contact or by contaminated article-to-person contact.  Sometimes an infected animal can be the source of scalp or body infections. Incubations time is generally 4 to 10 days.

Ringworm can be transmitted as long as the lesions (red patches or sores) are visible. Your child can return to school after his/her treatment has begun.

Ringworm lesions are generally round or oval in shape with a reddish rim and flat center; because the outside spreads, the center usually clears, leaving a more normal appearing skin area. Since other skin conditions can be similar, we urge you to take your child to the family physician, just to be sure.

Ringworm can be treated by an application of a fungicide (e.g., Tinactin or Micatin), but oral medication like Griseofulvin might be needed, it the ringworm doesn’t clear up. Oral medication can only be obtained with a prescription and is always necessary for cases on the scalp.

http://www.medicinenet.com/ringworm/article.htm