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In 1937, Rivers introduced a new set of postulates that were meant to replace those formulated by Koch.

However, I couldn't find an article (or other scientific literature) that describes why Koch's postulates weren't valid anymore.

Is there literature that explains why the old postulates were not valid anymore?

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    $\begingroup$ What's wrong with the reasons Rivers gives? $\endgroup$
    – Bryan Krause
    Jul 28 at 16:05
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If you read his 1937 article, Rivers himself makes a case for why Koch's postulates are too restrictive and that all the postulates need not be satisfied to confidently associate a pathogen with a disease. Here, I've highlighted excerpts and added my own emphasis.

[Koch's] dictum has had a profound influence on workers investigating infectious maladies and for many years an infectious agent was not accepted as the cause of a disease unless the postulates had been satisfied. With the development of the science of immunology, however, immunological reactions added much to the knowledge of the specific relation of microbes to disease, and now it is possible to bring excellent evidence that an organism is the cause of a malady without the complete satisfaction of the postulates.
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At the time when they were formulated Koch's postulates were essential for the progress of knowledge of infectious diseases; but progress having left behind old rules requires new ones which some day without doubt will also be declared obsolete. Thus, in regard to certain diseases, particularly those caused by viruses, the blind adherence to Koch's postulates may act as a hindrance instead of an aid.

Koch's postulates were published in 1890, but the first virus wasn't isolated until 1892 (at the earliest), so the postulates cannot be readily applied to diseases resulting from viral infections. That is not to say Koch's postulates were wrong, just that their blind application to all diseases may prevent the identification of novel etiological agents due to the requirement for isolation in pure culture. Rivers addresses this, and supplies less restrictive postulates for the characterization of viral infections.

It is obvious that Koch's postulates have not been satisfied in viral diseases. Moreover, it is equally evident that proof of the etiological significance of viruses has been obtained without their satisfaction. Such a statement, however, does not imply that certain conditons do not have to be met before the specific relation of a virus to a disease is established. The conditions are: (a) A specific virus must be found associated with a disease with a degree of regularity. (b) The virus must be shown to occur in the sick individual not as an incidental or accidental finding but as the cause of the disease under investigation.

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