Nearly 70 years ago millions of American parents made an audacious decision that would have global consequences. These parents agreed to enroll their first, second, and third graders in a human experiment of unprecedented scale and complexity—the first field trial of Jonas Salk’s polio vaccine.

Preliminary tests suggested the vaccine was effective and safe. Nevertheless, the trial was a leap into the unknown. Would the shots really work? What adverse effects might lurk in the shadows? And why would any sensible parent let their child be the one who found out?

Retired physician and historian of medicine Bruce Fye has a simple explanation: fear. Both he and his parents were “petrified of getting polio.” Fye, then a third grader in Mansfield, Ohio, was a Polio Pioneer, one of the 1.6 million children who in 1954 participated in a two-month testing blitz that spanned 44 states.

Polio was a disease of summer, its attack as sudden as thunderstorms and as ominous. Yet the disease was relatively uncommon: cases averaged 20,000 per year in the 1940s and early 1950s, peaking at 58,000 in the pandemic year of 1952. The death rate in 1952 was around 5%; estimates differ on the chance of paralysis, but it may have been as high as 25% of cases.