The Collaborative International Dictionary of English v.0.48:

Scarlet \Scar"let\, a.
   Of the color called scarlet; as, a scarlet cloth or thread.
   [1913 Webster]

   Scarlet admiral (Zool.), the red admiral. See under Red.
      -- Scarlet bean (Bot.), a kind of bean (Phaseolus
      multiflorus) having scarlet flowers; scarlet runner.

   Scarlet fever (Med.), a contagious febrile disease
      characterized by inflammation of the fauces and a scarlet
      rash, appearing usually on the second day, and ending in
      desquamation about the sixth or seventh day.

   Scarlet fish (Zool.), the telescope fish; -- so called from
      its red color. See under Telescope.

   Scarlet ibis (Zool.) See under Ibis.

   Scarlet maple (Bot.), the red maple. See Maple.

   Scarlet mite (Zool.), any one of numerous species of bright
      red carnivorous mites found among grass and moss,
      especially Thombidium holosericeum and allied species.
      The young are parasitic upon spiders and insects.

   Scarlet oak (Bot.), a species of oak (Quercus coccinea)
      of the United States; -- so called from the scarlet color
      of its leaves in autumn.

   Scarlet runner (Bot.), the scarlet bean.

   Scarlet tanager. (Zool.) See under Tanager.
      [1913 Webster]

WordNet (r) 3.0 (2006):

scarlet fever
    n 1: an acute communicable disease (usually in children)
         characterized by fever and a red rash [syn: scarlet
         fever, scarlatina]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "scarlet fever":
   African lethargy, Asiatic cholera, Chagres fever, German measles,
   Haverhill fever, 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, diphtheria, dumdum fever,
   dysentery, elephantiasis, encephalitis lethargica, enteric fever,
   erysipelas, famine fever, five-day fever, flu, frambesia,
   glandular fever, grippe, hansenosis, hepatitis, herpes,
   herpes simplex, herpes zoster, histoplasmosis, hookworm,
   hydrophobia, infantile paralysis, infectious mononucleosis,
   inflammatory rheumatism, influenza, jail fever, jungle rot,
   kala azar, kissing disease, lepra, leprosy, leptospirosis, loa loa,
   loaiasis, lockjaw, madness, malaria, malarial fever, marsh fever,
   measles, meningitis, milzbrand, mumps, ornithosis, osteomyelitis,
   paratyphoid fever, parotitis, parrot fever, pertussis, pneumonia,
   polio, poliomyelitis, polyarthritis rheumatism, ponos, psittacosis,
   rabbit fever, rabies, rat-bite fever, relapsing fever,
   rheumatic fever, rickettsialpox, ringworm, rubella, rubeola,
   scarlatina, schistosomiasis, septic sore throat, shingles,
   sleeping sickness, sleepy sickness, smallpox, snail fever,
   splenic fever, spotted fever, strep throat, swamp fever, tetanus,
   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