What three factors must be present for disease to develop, according to the disease triangle?

• A. Host must be susceptible, Pathogen must be present, Environment must be favorable.
• B. Host resistance must be complete, Pathogen absent, Environment favorable.
• C. Host must be tolerant, Pathogen present, Environment irrelevant.
• D. Host must be immune, Pathogen present, Environment unfavorable.

Answer: (A)

Disease develops when three factors come together: a host that can be infected (susceptible), a pathogen capable of causing disease, and an environment that supports infection and spread. The host doesn’t have to be perfectly susceptible—partial resistance can be overcome if the pathogen is present and the environment is favorable. If any one component is missing or unfavorable, disease is unlikely or greatly reduced. So the best statement is that the host must be susceptible, the pathogen must be present, and the environment must be favorable. The other ideas don’t fit because complete resistance isn’t required, the pathogen could be absent, or the environment could be unfavorable, all of which would hinder disease development.