Spread of Disease
Unlike most infectious diseases, the polio epidemic started in advanced and industrialized countries in North America and Europe. During the 19th and 20th centuries, these nations had already made significant improvements in health and hygiene, so many fatal diseases were becoming less common. (1)
These observations have lead health experts to believe that the infection was common in earlier times but that people were exposed and infected at young ages, providing partial immunity. As hygiene improved, young people were less likely to be exposed to the virus, allowing an epidemic to eventually break out. (2)
Early Timeline
(6)
(7)
Polio outbreaks were concentrated in the summertime and early autumn, and children were kept away from movie theaters, swimming pools, and crowded public spaces in order to avoid exposure to the poliovirus. Public hysteria was widespread because of the visable physical disabilities caused by polio. (3)
Researchers now know that the virus is transmitted from person to person through the fecal-oral route. In this process, fecal matter is taken into the mouth by either contaminated fingers or foods. Another way the virus is spread is from the water vapor of an infected person's breath. (4)
Individuals of the population who have not been vaccinated are less likely to contract the virus if most of the population has been vaccinated and has immunity to the virus.
This is because of herd immunity, which is a form of protection a community achieves whenever a large portion of a population has been vaccinated. (5)
Footnotes
1. Britannica School, s.v. "Poliomyelitis," accessed January 16, 2019, https://school.eb.com/levels/high/article/polio/60621.
2. Ibid.
3. Ibid.
4. Ibid.
5. "The Development of Polio Vaccines." In Science and Its Times, edited by Neil Schlager and Josh Lauer. Vol. 7. Detroit, MI: Gale, 2001.
6. The College of Physicians of Philadelphia. "History of Polio." The History of Vaccines. Accessed May 3, 2019. https://www.historyofvaccines.org/timeline/polio.
7. "Polio and Swimming Pools: Historical Connections." This History of Vaccines. Last modified June 28, 2012. Accessed May 3, 2019. https://www.historyofvaccines.org/content/blog/polio-and-swimming-pools-historical-connections.