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Herd immunity” is a settled concept in epidemiology. It occurs when “a large percentage of a population has become immune to an infection, whether through previous infections or vaccination, thereby providing a measure of protection for individuals who are not immune.”
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Herd mentality,” on the contrary, provides immunity from independent thought. It protects a population from thinking clearly by spreading a spirit of conformity. It increases a people’s docility, thus rendering them more susceptible to the blandishments of usurping authority.