Which type of pathogen causes disease only in a susceptible person?

• A. Primary Pathogen
• B. Opportunistic Pathogen
• C. Commensal Organism
• D. Strict Pathogen

Answer: (B)

The key idea is how a pathogen depends on the host’s defenses to trigger illness. An opportunistic pathogen causes disease only when the person is susceptible—for example, when the immune system is weakened, the protective barriers are breached, or other health stressors are present. In those situations, organisms that are usually harmless residents can invade, multiply, and cause infection. Think of Candida in immunocompromised patients, Pseudomonas infections in burn wounds, or Pneumocystis pneumonia in people with AIDS. In a healthy person, these organisms typically don’t cause disease because the defenses stay strong enough to keep them in check.

By contrast, organisms that are primary or strict pathogens are capable of causing disease in a healthy person upon exposure; they don’t rely on a compromised host. Commensal organisms are normally harmless residents of the body and can cause problems only if the balance is disrupted or the defenses fail.