Polio can be prevented by avoiding swimming pools, public gatherings, and summer heat exposure.

In the summer of 1952, nearly sixty thousand American children were diagnosed with poliomyelitis,the worst outbreak in the nation's history. Thousands were paralyzed. More than three thousand died. Public swimming pools closed. Movie theaters posted warnings. In cities and suburbs across the United States, parents kept children indoors during July and August, away from crowds, away from playgrounds, away from anywhere that the disease might be encountered. What no public health authority could tell them with confidence was exactly where or how polio spread, and the behavioral advice they offered,avoid heat, avoid crowding, avoid swimming,reflected that uncertainty more than any clear epidemiological understanding.

The association between summer and polio had been observed since the first American epidemics in Vermont in 1894. "Infantile paralysis," as it was then called, consistently peaked in August and September, then receded in autumn. The seasonal pattern encouraged speculation about causes that changed with temperature: contaminated water was a natural suspect, as were insects, since flies were more numerous in summer and were known to carry other diseases. Some physicians theorized that swimming pools concentrated and transmitted the virus; others focused on dust, on proximity, on anything that might explain why summer was so deadly. The public health advice issued during outbreaks reflected these theories: stay out of the water, stay home, avoid the summer heat.

Jonas Salk was working from a different premise. At the University of Pittsburgh, he and his team had spent years characterizing poliovirus,identifying its three distinct strains, determining that serum antibodies provided protection, learning to grow the virus reliably in monkey kidney tissue. Salk's approach was to kill the virus with formaldehyde and inject the inactivated particles as a vaccine, stimulating antibody production without any risk of infection from the vaccine itself. By 1952, as the epidemic peaked, his team was deep in development. The biology of poliovirus had become clear enough for the purpose of vaccination even before its transmission was fully understood.

What the pre-vaccine era had wrong was transmission, not the virus itself. Poliovirus spreads through fecal-oral contamination,ingested in water or food contaminated by infected feces, not inhaled from crowded spaces or acquired through cold water or summer heat. The seasonal peak reflected the summer habits of children: more outdoor activity, more hand-to-mouth contact, more shared food and water in contexts with less sanitation oversight. Swimming pools were a possible transmission route if their water was inadequately treated, but the danger was contamination, not the cold water itself. Heat was irrelevant to the virus.

The Francis Field Trial of 1954 enrolled 1.8 million American schoolchildren in the largest coordinated public health experiment ever attempted. Half received Salk's vaccine; the other half received a placebo; a third group was simply observed. On April 12, 1955,the tenth anniversary of Franklin Roosevelt's death, a choice of date that was not accidental, given Roosevelt's visibility as a polio survivor,Thomas Francis at the University of Michigan announced the results to more than five hundred reporters and a national radio audience. The vaccine was safe, effective, and potent. Against paralytic polio, it was 80 to 90 percent effective.

The swimming pools reopened the following summer. Parents who had spent years watching their children closely through July and August found themselves in a country that had a workable answer to the disease that had terrorized those summers. Polio cases fell by half within a year of the vaccine's introduction. The behavioral advice that had substituted for understanding,stay home, stay cool, stay out of the water,was retired along with the threat it had been unable to address.