The majority of infections are characterised by a mild fever often with vomiting or diarrhoea. Weakness or complete paralysis of any of the skeletal muscles appears in a minority of subjects, but this may develop rapidly. After a few days or weeks the weakness begins to improve and may continue to do so for one to two years.
It has recently been recognised that after an interval of at least 35 years after the acute infection, a condition known as the post-polio syndrome may develop. This is due to dying back of the peripheral nerve fibres which were damaged, but regenerated after the acute illness. This causes weakness, pain and fatigue in muscles which were previously affected. Post-polio syndrome occurs in around 50 per cent of those who have had polio, but in 80 per cent of these it is either static or only slowly progressive. Specialist advice about how to adapt the lifestyle to minimise the risk of the progression of this syndrome is often effective. Respiratory support, usually with a non-invasive ventilator system used at night, may be needed if the respiratory (breathing) muscles are affected. Post-polio syndrome should be distinguished from similar symptoms which may be due to degenerative changes in the joints or soft tissues, peripheral nerve entrapment, or aging.