Polio is an infectious disease caused by any one of three types of poliovirus. It is airborne, entering the body through the nose or mouth, and multiplying in the throat and intestines. The poliovirus may go on to invade the central nervous system, destroying or damaging the motor neurons (nerve cells that control muscles - see below). This can lead to muscle paralysis, affecting any part of the body, or death.

The polio epidemics of the 1950s and 1960s affected over 120,000 people in the UK. However, the introduction of the polio vaccination in the 1960s has largely eradicated the polio from the UK.

The polio illness can affect people in one of three ways:

• 95% of people have minor flu-like symptoms, such as nausea, vomiting, fever, sore throat and headache. People may not realise that they have polio.
• In around 5% of people, the virus gets into the nervous system. For most this will cause symptoms similar to meningitis (inflammation of the lining around the brain and spinal cord) or encephalitis (inflammation of the brain tissue) with high fever, stiff neck, back and muscle pain and headache - known as non-paralytic polio.
• In around 1 – 2% of people affected, the polio virus invades the motor neurons, leading to weakness, paralysis, muscle cramps and muscle pain. This is known as paralytic polio. Sometimes the poliovirus affects the brain stem, causing symptoms like breathing, swallowing or cardiovascular problems and facial weakness - known as bulbar polio.

Polio is not thought to affect the sensory nerves, so people can still feel pain. Polio does not affect hearing or eyesight.

In the early stages of polio where the poliovirus enters the nervous system, typically 95% of motor neurons may be infected. Some motor neurons will survive, others will die or are left damaged. Damage may be widely scattered throughout the body and so may not cause noticeable weakness. A muscle can usually still be used in everyday life and appear normal on muscle testing even when perhaps 50% of the motor neurons are lost or damaged, but it may be that the muscles are working at or near their maximum.

About half of the damaged motor neurons recover within the first month. During the following months or years, healthy or recovered motor neurons may sprout new nerve fibre branches to support some of the abandoned muscle fibres. Muscle fibres can grow larger to increase strength.

Tens of thousands of people in the UK survived polio and have gone on to lead active and independent lives, often after months or years in hospital and/or rehabilitation.

Although people were left with varying degrees of weakness, paralysis, fatigue, muscle pain, breathing and orthopaedic problems, these may have then stabilised. Others only had limited paralysis or appeared to have made a full recovery. Rehabilitation was seen as a positive regaining of strength and most people had no idea that some of these gains might be lost in the future or that there had been more damage than was outwardly obvious.

During the 1980s medical professionals began seeing more people with polio with increased weakness, fatigue and pain. It was not until the 1990s that there was enough evidence to suggest that these new symptoms were related to the effects of polio, and only recently have the reasons for the symptoms begun to be examined and understood.

We now know that people who had polio may develop new symptoms years later. For some, these new symptoms are the result of the original polio-related weakness, orthopaedic problems or other illness, but in others they are caused by Post Polio Syndrome (PPS).