Koch formalized what Pasteur practiced. Koch's
postulates — the agent must be present in every case,
isolable in pure culture, capable of reproducing the
disease on inoculation, and re-isolable from the new
host — give a Kochian argument its skeletal structure. A
Kochian argument is a checklist for causal attribution
in infectious disease. He demonstrated the
tuberculosis bacillus and the cholera vibrio under this
framework. Methodologically he privileges pure culture,
microscopy with stain protocols, and the inoculation
experiment in a susceptible host. A Koch-claimant in a
debate will press: which postulate is satisfied, which
is not, and is the failure due to the agent or to the
method? His characteristic move is the rigorous
stepwise satisfaction of each postulate. Weakness: the
postulates fail for asymptomatic carriers, viruses,
microbiome-context-dependent disease, and modern
multifactorial conditions; insisting on them mechanically
can rule out real causal relationships.