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.

any agent, whether a living organism or abiotic agent, that induces disease.

An organism which may cause disease.

A microorganism that causes disease; may be bacterial, viral, fungal, chlamydial, rickettsial, or parasitic.