Table 2.

Limitations of Koch's postulates.

Ignore the contribution of host, vector, and environment to disease susceptibility/response

Colonization state (e.g., +PPD skin test for tuberculosis in the absence of disease) violates Koch's second postulate

Many pathogens cannot be propagated on lifeless (cell-free) medium in the lab; these pathogens cannot fulfill Koch's third postulate

Viruses, parasites, uncultivated bacteria may not grow in pure culture

Host range restriction of pathogens

Do not consider the possibility of disease produced by a microbial community rather than a single pathogen

Not completely specific