It once affected mostly infants and children, but now is mostly seen in people over 15 years old. It is more common in the summer and fall. Adults and young girls are more likely to be infected, but infection in young boys is more likely to result in paralysis . Between 1840 and the 1950s, polio was a worldwide epidemic. Since the development of polio vaccines, the incidence is much reduced.
Outbreaks still occur, usually in non-immunized groups.
It may occur after recovery from a subclinical infection. When sitting, the person may need to support the body with the arms. Reflexes may be abnormal. The disorder may resemble encephalitis , and it may affect the cranial nerves and cause difficulty with facial expression, swallowing, chewing, and so on. It may also cause choking or difficulty breathing .