Rationale: Influenza in humans is caused by a virus, specifically the influenza virus, which is known for its ability to mutate and spread rapidly.
Option A, bacterium, is incorrect as bacteria do not cause influenza; they are responsible for different infections, such as strep throat.
Option C, fungus, is also not related to influenza; fungal infections typically affect the lungs but are distinct from viral infections like the flu.
Option D, protozoan, is incorrect as well; protozoa are single-celled organisms that cause diseases like malaria, not influenza. Thus, the only accurate choice is the virus.
