The Collaborative International Dictionary of English v.0.48:

Measles \Mea"sles\, n.; pl. in form, but used as singular in
   senses 1, 2, & 3. [D. mazelen; akin to G. masern, pl., and E.
   mazer, and orig. meaning, little spots. See Mazer.]
   [1913 Webster]
   1. (Med.) A contagious viral febrile disorder commencing with
      catarrhal symptoms, and marked by the appearance on the
      third day of an eruption of distinct red circular spots,
      which coalesce in a crescentic form, are slightly raised
      above the surface, and after the fourth day of the
      eruption gradually decline; rubeola. It is a common
      childhood disease.
      [1913 Webster +PJC]

            Measles commences with the ordinary symptoms of
            fever.                                --Am. Cyc.
      [1913 Webster]

   2. (Veter. Med.) A disease of cattle and swine in which the
      flesh is filled with the embryos of different varieties of
      the tapeworm.
      [1913 Webster]

   3. A disease of trees. [Obs.]
      [1913 Webster]

   4. pl. (Zool.) The larvae of any tapeworm (Taenia) in the
      cysticerus stage, when contained in meat. Called also
      bladder worms.
      [1913 Webster]

   German measles A mild contagious viral disease, which may
      cause birth defects if contracted by a pregnant woman
      during early pregnancy; also called rubella.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

rubella \ru*bel"la\, n. [NL., fr. L. rubellus reddish.] (Med.)
   An acute but mild viral infection characterized by a dusky
   red cutaneous eruption resembling that of measles, but
   attended by only mild respiratory problems or fever; --
   called also German measles. The infective virus is called
   Rubella virus, or Rubivirus. If contracted by a woman
   during the first several months of pregnancy, rubella may
   cause serious abnormalities in the fetus.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

rubella
    n 1: a contagious viral disease that is a milder form of measles
         lasting three or four days; can be damaging to a fetus
         during the first trimester [syn: German measles,
         rubella, three-day measles, epidemic roseola]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "rubella":
   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, rubeola, scarlatina,
   scarlet fever, 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