Cow pus. Vaccines started from cow pus. And it was that very fact that caused people to reject them over 200 years ago.

During the last decade, the most notable types of anti-vaxxers have claimed that autism and other "profound mental disorders" can be linked to vaccines. This theory was popularized by the British physician-resesearcher Andrew Wakefield in a paper he published in The Lancet, and has since been thoroughly discredited. The Lancet fully retracted the study in 2010.

Fast forward to today and vaccine-deniers of all stripes have emerged. Many groups, from the Amish of Ohio to Orthodox Jews in Brooklyn, have cited religious differences or medical concerns as their reason for not getting vaccinated.

But religious differences or concerns over side effects aren't the only reasons that people avoid vaccines. A survey conducted by Pew Research Center found that anti-vaxxers are more common among millennials rather than older generations. Many of these young adults believe that vaccinations should be a parental choice rather than a federal or state mandate.

Watch the video above to learn more about the weird and fascinating history of the anti-vaxxer movement.

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