What proportion of poliovirus infections are asymptomatic?

• A. About 70%
• B. About 10%
• C. More than 90–95% are asymptomatic
• D. About 50%

Answer: (C)

Most poliovirus infections do not cause symptoms. The virus typically stays in the gut and is shed in stool, with little systemic illness, so a large majority of infections remain inapparent. As a result, more than 90–95% of infections are asymptomatic. Only a small fraction cause mild illness, and an even smaller portion progress to meningitis or paralytic poliomyelitis. This high rate of silent infection is why poliovirus can spread quietly in communities and underscores the importance of vaccination to interrupt transmission. Options that imply much higher visible illness (around 70% or 50%) or a very low rate (around 10%) don’t match how poliovirus infections usually present.