Why are most humans infected with West Nile virus often asymptomatic?

The majority of humans infected with West Nile virus experience asymptomatic infections primarily due to the immune system's ability to effectively respond to the virus. The innate and adaptive arms of the immune system play a crucial role in recognizing and eliminating the virus before it can lead to symptomatic illness. Many individuals possess pre-existing immunity from prior exposure to related viruses, which can further enhance their response to West Nile virus. This asymptomatic nature is typical for many viral infections, where a robust immune response neutralizes the pathogen without causing noticeable disease.

While other factors such as exposure rates and vector control efforts may influence the overall incidence of the virus in the population, they do not directly account for the asymptomatic nature of the infections themselves. Similarly, while a higher viral load could potentially lead to symptomatic disease, the majority of infections do not result in high enough viral levels to overwhelm the immune response in most individuals.