Pathogen. Definition. noun, plural: pathogens. An agent causing disease or illness to its host, such as an organism or infectious particle capable of producing a disease in another organism.Supplement. Pathogens are mostly microscopic, such as bacteria, viruses, protozoa, and fungi, thriving in various places such as air, dust, surfaces, soil, etc.athogen. Definition. noun, plural: pathogens. An agent causing disease or illness to its host, such as an organism or infectious particle capable of producing a disease in another organism.