In Koch's postulates, what must be true about the agent in every case?

• A. The agent must be present in every case.
• B. The agent must be easily cultivable in all media.
• C. The agent must be associated with disease but not necessarily present.
• D. The agent must be found in the host but not necessarily in all cases.

Answer: (A)

The main concept is that a disease-causing organism must be present in every case of the disease. This consistent presence is what ties the agent to the illness rather than just being occasionally associated. In Koch's framework, proving causation relies on showing the organism can be found in all affected individuals, then isolating it in pure culture, demonstrating that introducing it into a susceptible host reproduces the disease, and finally re-isolating the same organism from that host. If the agent isn’t present in every case, the link to causation weakens because the disease could arise from other factors or pathogens. Of course, practical limits exist—some pathogens are hard to culture, or infections may be asymptomatic or involve complex interactions—but the criterion in question is that the agent must be found in every case.