The Collaborative International Dictionary of English v.0.48:

kissing disease \kiss"ing dis*ease"\
   Infectious mononucleosis; -- so called because often spread
   by kissing.
   [PJC]

WordNet (r) 3.0 (2006):

kissing disease
    n 1: an acute disease characterized by fever and swollen lymph
         nodes and an abnormal increase of mononuclear leucocytes or
         monocytes in the bloodstream; not highly contagious; some
         believe it can be transmitted by kissing [syn: infectious
         mononucleosis, mononucleosis, mono, glandular fever,
         kissing disease]