October 24 is World Polio Day. For those of us who lived through the polio epidemics—or who had a family member, friend, classmate or neighbor affected by this terrible disease—the word polio means much more than a name in a history book.

We remember.

We remember children who suddenly could not walk. We remember leg braces, crutches and wheelchairs. We remember hospital wards filled with frightened children and parents. We remember the iron lung. We remember families waiting anxiously for news about whether their child would walk again.

And we remember something else: polio did not affect only children.

Children were particularly vulnerable, but teenagers and adults could also become ill. Some developed severe paralysis. Others experienced a much milder illness and may never have realized that they had been infected with poliovirus.

Today, most Americans know the word polio for a very different reason: they were vaccinated against it.

That is one of the great public-health successes of our lifetime.

But there is a part of the polio story that is still not widely understood.

Polio can have a Part Two

Many polio survivors who appeared to have recovered decades ago are now experiencing what is known as Post-Polio Syndrome (PPS).

PPS can occur many years after the original infection. Survivors may experience increasing muscle weakness, fatigue, muscle and joint pain, problems with walking, breathing or swallowing, sleep difficulties and increased difficulty performing activities they once managed easily.

For some survivors, it can feel as though polio has returned.

It hasn't.

The original poliovirus infection is not returning. Rather, researchers believe that the enormous demands placed on nerve cells and muscles that compensated for the original damage may contribute to new weakness and other problems decades later.

In other words, for many survivors, polio has a Part Two.

The exact number of people living with Post-Polio Syndrome is difficult to determine. Millions of people worldwide survived polio, and many survivors are now older adults. Some may not even realize that symptoms they are experiencing today could be related to their childhood illness.

That is why telling the story remains so important.

We must remember the epidemics

During the major U.S. polio epidemics of the first half of the 20th century, tens of thousands of Americans were paralyzed by polio. The 1952 epidemic alone was one of the worst in U.S. history, with more than 57,000 reported cases and more than 3,000 deaths.

Then came a medical breakthrough that changed everything.

In 1955, the Salk polio vaccine was licensed in the United States.

Vaccination dramatically changed the course of polio in America. The number of cases fell rapidly, and the disease that once terrified American families became increasingly rare.

But rare does not mean impossible.

The danger of forgetting

Polio remains a threat in parts of the world, and poliovirus can travel across borders. If vaccination coverage declines, communities can become vulnerable to outbreaks.

That is why World Polio Day is not simply a day to remember the past.

It is a day to protect the future.

Herd immunity is not a magic number that guarantees protection everywhere. The vaccination coverage needed to prevent sustained transmission depends on the disease and the population. Because poliovirus is highly contagious, maintaining very high vaccination coverage is essential.

The lesson of the polio epidemics is simple:

We do not want to learn about polio by experiencing it again.

Imagine having to explain to today's children what an iron lung was. Imagine telling them that a child could go to bed seemingly healthy and wake up unable to move. Imagine explaining why schools sometimes closed, swimming pools were avoided, and parents feared letting their children play with other children.

We don't want to go back there.

Remember the survivors

World Polio Day should also be about the people who survived.

They grew up.

They went to school. They went to work. They married. They raised families. They became teachers, doctors, lawyers, scientists, businesspeople, artists, parents and grandparents.

Some used braces.

Some used crutches.

Some used wheelchairs.

Some walked with canes.

And some had disabilities that were invisible to everyone around them.

They didn't simply survive polio. They built lives after polio.

Today, many of those survivors are dealing with the unexpected second chapter of their disease: Post-Polio Syndrome.

Their stories deserve to be heard.

Don't let history repeat itself

World Polio Day gives us an opportunity to remember what polio did—and to recognize what vaccination accomplished.

Vaccines don't erase history. They allow us to learn from it.

The generations growing up today may never see an iron lung. They may never know a child wearing a full leg brace because of polio. They may never understand why parents once feared the arrival of summer.

That is a good thing.

But if we forget why those things disappeared, we risk taking that protection for granted.

So on October 24, remember the children who were paralyzed.

Remember the families who lived through the epidemics.

Remember the survivors who are now aging with Post-Polio Syndrome.

And remember the vaccine that changed the story.

Polio is part of our history. Let's make sure it never becomes part of our future again.