Someone who opposes the public health practice of vaccination.
Almost always, the reasons for their opposition are rooted in incorrect beliefs about vaccination. This can range from simply having fallen for urban legends about vaccine ingredients or about purported side effects of vaccines, to holding an elaborate conspiracy theory
where literally every single doctor and researcher in the whole of the world who has personally studied vaccines and said 'No, they're much safer than the diseases they protect against' has been bought off.
Thanks to crank magnetism
, anti-vaxxers are also frequently believers in chemtrails, the New World Order, HAARP weather modification, et cetera.
See flat earther
That anti-vaxxer started by saying "I just don't think the government should be able to dictate my medical treatment" - but within a few minutes she was ranting about how Pasteur supposedly recanted germ theory on his deathbed and Big Pharma's been covering it up ever since.