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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.


Pathogens are mostly microscopic, such as bacteria, viruses, protozoa, and fungi, thriving in various places such as air, dust, surfaces, soil, etc.

Not all bacteria are pathogens, in fact most of them are harmless and only a few are pathogenic. Examples of pathogenic bacteria are Mycobacterium tuberculosis (causing tuberculosis), Streptococcus penumoniae (causing pneumonia), Shigella, Campylobacter and Salmonella (causing foodborne illnesses).

Examples of diseases caused by pathogenic viruses are smallpox, influenza, mumps, measles, chickenpox and rubella.

Word origin: from Greek pathos, suffering/emotion, and gene, to give birth to.
Related forms: pathogenic (adjective), pathogenesis (noun), pathogenous (adjective), pathogeny (noun).
Related phrases: opportunistic pathogen.

Synonym: infectious agent, germ.
See also: disease, virulence.