The Collaborative International Dictionary of English v.0.48:

Ringworm \Ring"worm"\, n. (Med.)
   A contagious affection of the skin due to the presence of a
   vegetable parasite, and forming ring-shaped discolored
   patches covered with vesicles or powdery scales. It occurs
   either on the body, the face, or the scalp. Different
   varieties are distinguished as Tinea circinata, Tinea
   tonsurans, etc., but all are caused by the same parasite (a
   species of Trichophyton).
   [1913 Webster]

WordNet (r) 3.0 (2006):

Trichophyton
    n 1: a genus of fungus of the family Moniliaceae; causes
         ringworm and favus [syn: Trichophyton, genus
         Trichophyton]