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):

ringworm
    n 1: infections of the skin or nails caused by fungi and
         appearing as itching circular patches [syn: tinea,
         ringworm, roundworm]

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "ringworm":
   African lethargy, Asiatic cholera, Chagres fever, German measles,
   Haverhill fever, acne, acne vulgaris, acute articular rheumatism,
   ague, alkali disease, amebiasis, amebic dysentery, anthrax,
   bacillary dysentery, bastard measles, black death, black fever,
   blackwater fever, breakbone fever, brucellosis, bubonic plague,
   cachectic fever, cerebral rheumatism, chicken pox, cholera, cowpox,
   dandy fever, deer fly fever, dengue, dengue fever, dermamycosis,
   dermatitis, dermatosis, diphtheria, dumdum fever, dysentery,
   eczema, elephantiasis, encephalitis lethargica, enteric fever,
   epithelioma, erysipelas, erythema, exanthem, famine fever,
   five-day fever, flu, frambesia, glandular fever, grippe,
   hansenosis, heat rash, hepatitis, herpes, herpes simplex,
   herpes zoster, histoplasmosis, hives, hookworm, hydrophobia,
   impetigo, infantile paralysis, infectious mononucleosis,
   inflammatory rheumatism, influenza, itch, jail fever, jungle rot,
   kala azar, kissing disease, lepra, leprosy, leptospirosis, lichen,
   lichen primus, loa loa, loaiasis, lockjaw, lupus, lupus vulgaris,
   madness, malaria, malarial fever, marsh fever, measles, meningitis,
   miliaria, milzbrand, mumps, ornithosis, osteomyelitis,
   paratyphoid fever, parotitis, parrot fever, pemphigus, pertussis,
   pneumonia, polio, poliomyelitis, polyarthritis rheumatism, ponos,
   prickly heat, pruigo, pruritus, psittacosis, psora, rabbit fever,
   rabies, rat-bite fever, relapsing fever, rheumatic fever,
   rickettsialpox, rubella, rubeola, scabies, scarlatina,
   scarlet fever, schistosomiasis, septic sore throat, shingles,
   skin cancer, sleeping sickness, sleepy sickness, smallpox,
   snail fever, splenic fever, spotted fever, strep throat,
   swamp fever, tetanus, tetter, thrush, tinea, trench fever,
   trench mouth, tuberculosis, tularemia, typhoid, typhoid fever,
   typhus, typhus fever, undulant fever, vaccinia, varicella, variola,
   venereal disease, viral dysentery, whooping cough, yaws,
   yellow fever, yellow jack, zona, zoster