Koch's postulates

Koch's postulates are criteria for linking a microorganism to a disease through its presence in diseased hosts, isolation, experimental infection, and reisolation.

Causal inference: Reasoning that identifies whether one factor produces a change in another. The postulates are an early experimental framework for inferring microbial causation.
Robert Koch: A German physician and microbiologist who established methods for identifying bacterial pathogens. Koch developed the experimental methods associated with the postulates.
Mycobacterium tuberculosis: A slow-growing bacterium that causes tuberculosis in humans. Its identification as the tuberculosis agent became a landmark application of Koch's methods.
Asymptomatic carrier: A person or animal that carries an infectious agent without showing symptoms. Carriers violate the expectation that a suspected pathogen appears only in diseased hosts.
Microbial culture: The growth of microorganisms in controlled laboratory conditions. Culturing separates a suspected pathogen from other organisms in a diseased host.
Friedrich Loeffler: A German bacteriologist who formulated criteria for linking microorganisms to infectious diseases. Loeffler's 1884 formulation helped shape the criteria later associated with Koch.
Bacillus anthracis: A spore-forming bacterium that causes anthrax in humans and other animals. Koch's experiments linked this bacterium to anthrax using isolation and infection.
Opportunistic pathogen: A microorganism that causes disease mainly when host defenses or barriers are compromised. Its presence alone does not predict disease, complicating the postulates' causal test.
Pure culture: A laboratory culture containing one microbial species or strain. The isolation step aims to test a pathogen apart from accompanying microbes.
Germ theory of disease: The theory that microorganisms can cause infectious diseases. The postulates supplied experimental support for germ theory's claims about specific diseases.

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