An agent of disease. A disease producer. The term pathogen most commonly is used to refer to infectious organisms. These include bacteria (such as staph), viruses (such as HIV), and fungi (such as yeast). Less commonly, pathogen refers to a noninfectious agent of disease such as a chemical.
The term pathogen was devised about 1880 and was compounded from patho- meaning disease + -gen indicating a producer. Hence, a disease producer.