Jonas Salk: Poliomyelitis struck first at infants. That was why it was initially called infantile paralysis. But as hygienic conditions improved, the virus spread in the population in a different way than it did when hygienic conditions were poor. When hygienic conditions were poor, many infants died of diarrheal diseases. In the course of the infection that was spread that way, perhaps by exposure to sewage and unclean environments, they would very likely acquire the poliomyelitis virus infection, which, if it occurred in the first six months of life, would protect them against paralysis because of maternal antibody. After maternal antibody was lost, and the infection was acquired after six months of life, then paralysis would ensue. So at first it was an infection that would occur within the first six months to a year of life, or two or three years of life. But as time went on and hygienic conditions improved, they were spared the infantile infection, but were exposed later when paralysis could occur. By the time the early 1950s appeared, about 25 percent of paralytic cases occurred in those 21 years of age and older. In fact, Franklin Roosevelt, who was a President of the United States, was paralyzed at the age of 39.
And so at that time the age distribution had changed. It was a disease that was spread less by water supply or by exposure to fecal contamination. It was spread now more by pharyngeal spread, in the family context or amongst playmates — the secretions of the nose and throat. Because the virus would enter the blood stream, it would multiply in the intestines, enter the blood stream, and then it would enter into the nervous system, the spinal cord — paralyze — but it would also appear in the throat. And then, it could spread that way in schools, and amongst playmates. Well that’s how it was spread. As far as the incidence in the early 1950s — in the five years before the vaccine was available in ’55 — about 25,000 cases occurred annually. The highest incidence was over 50,000 cases in 1953, I think it was. And this gives you some measure of the concern because it would crop up anywhere, at any time, without any forewarning.