Basically, there was a viral rumor spread through social media that vaccination causes autism, or that it can used by the government to track people (and about 50 other hypotheses) none of which have no basis in the official science published through repeated trials and testing. Over the years, there have been a progressively political and religious slant to these anti-vax beliefs as well.

TLDR: Some parents refuse to vaccinate their kids, putting them at risk for innumerable easily preventable diseases, because they believe there a chance the vaccines will harm the kid. Yet, all scientific studies conducted have repeated concluded vaccines are harmless and help people live healthier lives.