4 years ago
5 years ago
7 years ago
The shots have cut disease rates so low that some parents underestimate the threat, doctors say
But small pockets of world still have cases of crippling disease, officials add
8 years ago
But definitive cause of polio-like disease remains unproved, researchers say
9 years ago
Almost 2 million American kids stuck out their arms for an unheard-of test as Jonas Salk set out to prove his vaccine worked