When the Salk vaccine first became available in 1955 there was an incident where some vaccine made by Cutter Labs in Berkeley, CA was improperly produced and caused some people (mostly children) to get polio. People at the time were so afraid of polio that they went ahead and got their kids vaccinated even after this happened. Compare that to today, when some people are so paranoid that they deliberately infect their kids instead of protect them with vaccines that have been shown to be very safe.