Which statement best captures a limitation of Koch's postulates when applied to modern pathogens?

• A. They apply equally well to all pathogens.
• B. They struggle with chronic diseases, viruses, prions.
• C. They require no experimental inoculation.
• D. They are based solely on observational data.

Answer: (B)

Koch's postulates are built on the idea that a single, cultivable organism causes a disease and that this can be demonstrated by isolation and reproduction in a susceptible host. That framework doesn’t fit well with several modern pathogens. Viruses, for example, require living host cells to replicate and aren’t readily grown as a pure, independent organism in the way bacteria are, so the standard steps of isolation and culture are not straightforward. Prions complicate things further because they are infectious proteins with no nucleic acid, so you can’t culture them or reproduce disease in the classic sense using a purified organism. Chronic diseases pose another challenge: they often involve long latent periods, multifactorial etiologies, and may persist even after the infectious agent is cleared or may require host and environmental factors to trigger disease, making a direct, single-cause demonstration unreliable. Because of these gaps, the statement captures the main limitation: Koch’s postulates struggle with chronic diseases, viruses, and prions. In practice, clinicians rely on a combination of epidemiology, molecular evidence, and mechanistic data rather than the original postulates alone.