On February 23, 1954, the polio vaccine, developed by Jonas Salk, commenced clinical testing… before the introduction of the vaccine in the mid-50’s, regular polio outbreaks occurred everywhere, even developed countries like the USA and northern Europe.
The virus is transmitted through fecal matter, so swimming pools, improper sanitation (like washing after using the bathroom), babies diapers and other sources moved the virus.
Almost all of the transmission was through casual contact, not improper sanitation (at least since the advent of a modern sanitation system in the USA starting in the late 1920’s).
About 95% of individuals infected are asymptomatic (pdf), so they appear healthy but are shedding viruses to infect other people.
Of the 5% who are symptomatic, about 10% of them eventually progress to the paralytic version of the disease. In other words, approximately 0.5% of those infected were paralyzed. One of the tropes of the anti-science/anti-vaccine world is that this is a small number. Except it isn’t. Out of 5 million children who might be infected every, approximately 25,000 children a year might progress to the paralytic version of the disease, and some of them would die.
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