Koch's postulates are problematic for which types of diseases?

• A. Acute bacterial infections
• B. Chronic diseases, viruses, prions
• C. Fungal infections
• D. Parasitic infestations

Answer: (B)

Koch's postulates hinge on identifying a single microorganism that is found in every case of a disease, can be isolated in pure culture, and will cause the disease when introduced into a healthy host, with the organism then being re-isolated from the newly diseased host. This approach fits well with classic acute bacterial infections where the agent can be grown in the lab and reliably transmitted.

It becomes problematic for viruses because they require living host cells to replicate; you cannot obtain a pure, cell-free culture of a virus in the way Koch envisioned. The same issue applies to prions, which are infectious proteins lacking nucleic acids and cannot be cultured or isolated using standard microbiologic methods. For chronic diseases, the difficulty lies in the fact that they are often not caused by a single, identifiable microbial agent. They may be multifactorial, involve latent or persistent infections, or arise from noninfectious processes. Because of these features, the strict sequence of discoveries demanded by Koch’s postulates cannot be fulfilled.

So, the diseases where Koch's postulates are not well suited include chronic diseases, viruses, and prions.