Koch's postulates are guidelines to demonstrate that an agent causes disease. Which statement is one of the postulates?

• A. The agent must be present in every case.
• B. The agent must be recovered from the environment and identified.
• C. The agent must be isolated and grown in pure culture.
• D. The agent must cause disease upon inoculation into a susceptible animal.

Answer: (A)

Demonstrating causation relies on showing a consistent, exclusive association between a specific agent and the disease. Koch's first postulate states that the organism must be found in all cases of the disease and not found in healthy individuals, linking the presence of the agent directly to the disease across all affected individuals. That exact idea is captured by the statement that the agent must be present in every case, making it the best choice. In the rest of Koch's framework, the agent should be isolable in pure culture, must cause disease when introduced into a susceptible host, and should be recoverable again from the newly diseased host to confirm identity. Recovering the agent from the environment, while useful in understanding reservoirs, is not one of the original postulates.