The Collaborative International Dictionary of English v.0.48:

Pathogen \Path"o*gen\, Pathogene \Path"o*gene\, n. [See
   Pathogenic.] (Biol.)
   Any microorganism which causes disease; a pathogenic
   organism; an infectious microorganism; a bacterium, virus, or
   other agent which can cause disease by infection; -- opposed
   to zymogene. The spelling pathogene is now archaic.
   [1913 Webster + PJC]

WordNet (r) 3.0 (2006):

pathogen
    n 1: any disease-producing agent (especially a virus or
         bacterium or other microorganism)