Mark died on July 4, 1999, from post-polio syndrome.

Ultimately he also was affected by post-polio syndrome much later in life.

Only people who have had polio can get post-polio syndrome.

But having post-polio syndrome doesn't mean that you have polio again.

Researchers are studying other possible causes of post-polio syndrome.

Symptoms of post-polio syndrome tend to show up very slowly.

Doctors identify post-polio syndrome by checking your medical history and current signs of illness.

You are more likely to get post-polio syndrome if you:

The cause was a heart attack after several years of partial paralysis from post-polio syndrome, his family said.

With post-polio syndrome, he said, the one sure thing is that, over time, muscles will weaken.