A pathogen is any disease-producing agent, especially a virus, bacterium, or other microorganisms. Pathogens can be spread through skin, bodily fluids, airborne particles, and touching a contaminated surface.

In biology a pathogen (Greek: πάθος pathos "suffering", "disease" or "passion" and -γενής -genēs "producer of" "born of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.