Ring worm or tinea is superficial eruption of the skin caused by a fungus, chiefly Microsporum, Trichophyton, or Epidermophyton. The most common affected area is the feet, but any area of the skin may be affected, including the scalp and nails.

Fungi are present on the bodies of most persons, but some individuals are more resistant to fungus invasion than others.

It is spread by direct contact with an infected person or animal, contact with soil or by indirect contact with items contaminated by the fungus (clothing, towels, bedclothes and toilet articles). On the scalp, ringworm begins in the form of a pimple sore, which then spreads into a ring shape. Hair becomes brittle, breaking easily and falling out, leaving bald spots on the scalp. On the body, ringworm may first appear as red or pink, flat or slightly raised, patched on the skin. Infection in the nails usually begins at the site of an injured nail and may spread to the other nails. Infected nails become thick, pitted, grooved and abnormal in shape and colour.

Although specific figures are not available, ringworm is a frequent problem in most countries, particularly where personal and domestic hygiene are poor.

Primary source : http://www.lenntech.com/library/diseases
Source : http://en.wikipedia.org/wiki/Tinea