Pathogen

n.

1.

(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.

Noun

1.

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

infectious agent, infective agent, microorganism