Which of the following is an infectious disease agent?

• A. Cancer
• B. Diabetes
• C. Mouse hepatitis virus
• D. All of the above

Answer: (C)

An infectious disease agent is a biological entity capable of causing infection in a host, such as a virus, bacterium, fungus, parasite, or prion. Cancer and diabetes are diseases themselves, not infectious agents that spread infection. Cancer involves uncontrolled cell growth, and diabetes is a metabolic disorder affecting glucose regulation. The mouse hepatitis virus is a virus that can infect mice and cause disease, so it is the infectious disease agent among the options.