What percentage of those infected with West Nile Virus develop severe disease?

• A. 20%
• B. 5%
• C. 50%
• D. 80%

Answer: (A)

The main idea here is how West Nile virus infections present across people who are infected. Most infections don’t cause symptoms at all, but a portion do develop illness, and a very small portion progress to serious neurological disease. In many summaries, about one-fifth of those infected go on to have a clinical illness such as West Nile fever, with symptoms like fever, malaise, and headaches. The scary, severe neuroinvasive disease—such as meningitis or encephalitis—happens much less often, and the risk is highest in older adults. So the figure around 20% reflects the share that become clinically ill, not the share that become severely ill. The other options describe proportions that don’t match the typical distribution, since severe disease is far rarer than 20% and the numbers 50% or 80% would imply many more cases becoming severe than actually do. If the question were specifically about severe disease, the answer would be a much smaller fraction.