Beating polio once may not be enough. A report from the March of Dimes says millions of polio survivors are at risk from a slow-moving, little understood secondary illness called post-polio syndrome, PPS.

The condition can strike anywhere from 15 to 40 years after their original illness - leaving sufferers using wheelchairs or ventilators for the rest of their lives.

Symptoms include muscle weakness that grows gradually worse, severe fatigue and pain in muscles and joints. This is often accompanied by decreased muscle endurance during activities.

According to the March of Dimes, too few doctors recognize PPS. And even if they do, no cure exists.

As many as 40 per cent of polio survivors may get PPS, according to some estimates.

Nerve fibres wear out

The causes of PPS are not thoroughly understood, but there are some clues.

Dr. Lewis Rowland at Columbia University College of Physicians and Surgeons says when patients first have polio, they can lose up to 60 or 70 per cent of their motor nerve cells.

The surviving nerve cells then find muscle fibres that still work and attach to them, restoring function. After 15 to 40 years Rowland says, the ability to maintain function seems to be lost.

Some say this may be because these motor nerves have been supplying many more muscle fibres than originally intended and get worn out.

Whatever the cause, the most widely accepted theory is that PPS results from the degeneration of these motor nerves.