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Fifty years ago, Jonas Salk announced he had developed a safe and effective vaccine against the most dreaded childhood disease of the 20th century. Others may deserve as much credit for their roles in the conquest of polio, but no one will be remembered more fondly than Salk for the hope he gave to parents and children around the world.

Iron lungs. Leg braces. Isolation wards. For anyone 50 or older, these words can bring back flashes of dread. Younger generations—with more than a dozen immunizations under their belts—can't remember the years when polio epidemics were a terrifying annual event. Parents throughout the Western Hemisphere no longer have to fear an infection that could start like a summer cold but end with their child dead or paralyzed for life. Most are unaware that, half a world away, polio still ravages children in Africa and southern Asia.

Poliomyelitis, known popularly as polio, is a highly infectious viral disease transmitted through fecal-oral routes, often through contaminated food or water. Though polio can strike anyone at any age, children are especially at risk. The virus primarily infects the intestines, without causing serious illness, but sometimes it attacks nerve cells of the central nervous system. Symptoms vary from mild, temporary paralysis to extensive paralysis resulting in permanent quadriplegia. In its most severe form, bulbar polio, the virus attacks the brain stem, destroying the motor neurons that tell the body how to swallow, speak, and breathe. Without respiratory support, a patient with this type of polio usually dies.

In her book Patenting the Sun: Polio and the Salk Vaccine, Jane Smith recalls a chilling scene from polio's tragic past:

It struck lightly at first—a summer cold, a headache, a mild fever that was scarcely more than the flush of playing outdoors on a steamy day. Then suddenly there was the faint crash of a small body falling, the cry of terror. "Mama, I can't move!" "My head, Pa, I can't lift my head!" There was the scream of pain as the little arms and legs twisted inward on themselves, or the most fearful sound of all, the choking rasp that came when the lungs forgot how to pump and the throat how to swallow, when before your eyes the baby grew still and blue and cold.

Parents watched helplessly as yesterday's healthy child was rushed to the hospital and encased in a claustrophobic iron lung. Mothers and fathers could only pray that their child's body would recover and remember how to breathe again. Little legs that used to climb trees and play hopscotch became twisted and atrophied, requiring the use of heavy metal braces, crutches, or wheelchairs for mobility.

No wonder older generations shiver at these memories.

There is still no cure for polio. But it can be prevented. This year marks the 50th anniversary of Jonas Salk's injectable killedvirus polio vaccine (IPV). Along with Albert Sabin's later oral live-virus version, the Salk vaccine made it possible to conquer this dreaded disease in every country of the Americas (see sidebar p. 15). Today, children and parents around the world rest easier for Salk's and Sabin's achievements.

The first known scientific description of polio was recorded in 1789 by British physician Michael Underwood. He reported a strange disease that seemed to target children, leaving them with residual paralysis. Polio continued to resurface in pockets each year, but it wasn't until the early 20th century that the number of paralytic cases reached epidemic proportions.

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