What proportion of people infected with West Nile Virus typically develop West Nile fever?

• A. About 20%
• B. About 50%
• C. About 80%
• D. About 5%

Answer: (A)

West Nile virus infection has a wide range of clinical presentations, but the common pattern is that most people don’t develop noticeable illness. About one in five infected individuals go on to develop West Nile fever, which is a self-limited, flu-like illness with fever, fatigue, headache, and sometimes muscle pains. The remaining roughly 80% are asymptomatic, and a small fraction may develop more severe disease affecting the nervous system. So the proportion that develops West Nile fever is about 20%. The other options don’t fit typical infection patterns because they would overstate or understate how often fever appears in primary infection.