Polio is a serious disease caused by a virus that can damage the nerves controlling muscles, sometimes leaving people unable to walk or even breathe on their own. The disease mainly affected children, which is why it's also called infantile paralysis.

Before the 1950s, polio terrified parents in many countries. Every summer, when the virus spread most easily, thousands of children caught polio. Most recovered completely, but some became paralyzed in their legs, arms, or the muscles needed for breathing. Patients who couldn't breathe on their own sometimes lived inside large machines called iron lungs that helped them breathe.

Then in 1955, Dr. Jonas Salk developed a vaccine that helped prevent polio. Within a few years, millions of children received the vaccine, and polio cases dropped dramatically. Today, thanks to vaccination campaigns worldwide, polio has been eliminated from most countries, although it still exists in a few parts of the world.