The Collaborative International Dictionary of English v.0.48:

Contagious disease \Con*ta"gious dis*ease"\ (Med.)
   A disease communicable by contact with a patient suffering
   from it, or with some secretion of, or object touched by,
   such a patient. Most such diseases have already been proved
   to be germ diseases, and their communicability depends on the
   transmission of the living germs. Many germ diseases are not
   contagious, some special method of transmission or
   inoculation of the germs being required.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

contagious disease
    n 1: any disease easily transmitted by contact [syn: contagious
         disease, contagion]