1880, a back-formation from pathogenic.
pathogen path·o·gen (pāth'ə-jən)
n. 
 An agent that causes disease, especially a living microorganism such as a bacterium, virus, or fungus.
| pathogen   (pāth'ə-jən)     An agent that causes infection or disease, especially a microorganism, such as a bacterium or protozoan, or a virus. See Note at germ.  | 
A disease-causing agent. Microorganisms, viruses, and toxins are examples of pathogens.