Source : Webster's Revised Unabridged Dictionary (1913)

Infectious disease \In*fec"tious dis*ease"\
   (a) Any disease caused by the entrance, growth, and
       multiplication of bacteria or protozoans in the body; a
       germ disease. It may not be contagious.
   (b) Sometimes, as distinguished from contagious disease, such
       a disease communicated by germs carried in the air or
       water, and thus spread without contact with the patient,
       as measles.

Source : WordNet®

infectious disease
     n : a disease transmitted only by a specific kind of contact