Source : Webster's Revised Unabridged Dictionary (1913)

Diphtheria \Diph*the"ri*a\, n. [NL., fr. Gr. ? leather (hence
   taken in the sense of membrane): cf. ? to make soft, L.
   depsere to knead.] (Med.)
   A very dangerous contagious disease in which the air
   passages, and especially the throat, become coated with a
   false membrane, produced by the solidification of an
   inflammatory exudation. Cf. {Group}.

Source : WordNet®

diphtheria
     n : acute contagious infection caused by the bacterium
         Corynebacterium diphtheriae; marked by the formation of a
         false membrane in the throat and other air passages
         causing difficulty in breathing