What is the cause of infection, a microbe that causes a disease?

• A. Host
• B. Reservoirs
• C. Communicable Disease
• D. Pathogen

Answer: (D)

The main idea is identifying the agent that causes disease—the pathogen. A pathogen is a microorganism such as a bacterium, virus, fungus, or parasite that invades the body and leads to illness. It is the disease-causing agent itself, which is why it’s described as the pathogen.

The other terms describe roles or characteristics in disease development but aren’t the cause. A host is the person or organism that harbors the pathogen. A reservoir is a place where the pathogen can live and persist, sometimes without causing illness in that location. A communicable disease is one that can spread from one host to another; it refers to transmission, not the agent itself.