The Collaborative International Dictionary of English v.0.48:

Diphtheritic \Diph`the*rit"ic\, a. (Med.)
   1. Pertaining to, or connected with, diphtheria.
      [1913 Webster]

   2. Having characteristics resembling those of diphtheria; as,
      diphtheritic inflammation of the bladder.
      [1913 Webster]