A very long time and very smart friend of ours has now made "anti-vax" their identity. Their Facebook feed is just one long string of anti-vax posts. They no longer work for animals, as they had for decades, because they won't get vaccinated.
It seems it started in 2016 with their capture by the Bernie Bros contingent. It wasn't Senator Sanders' ideas (Denmark is happier than the United States; if it was politically possible, we would gladly support those ideas). Rather, it was the anti-Clinton and then anti-establishment posts. Then it morphed into acceptance of more and more "everyone is out to get me and my small group who know the truth" conspiracies. (I wrote this last year in an obviously unsuccessful attempt to influence them.)
The problem is that this evolution is entirely understandable. You read more and more, deeper and deeper, until it is all of a piece and no one else can be believed. This would have happened to just about anyone with the same background and the same media consumption (e.g., study of YouTube's radicalizing algorithm).
The same is true for people who grew up listening to Rush Limbaugh at home and then started watching Fox News. Racists. Sexists. Anti-LGBTQ fanatics. Killers. They all believe that because that is what got into their brain.
It is, really, the same for people growing up in a religion. What you believe is what you see, and vice versa. Our brains are not rational machines, but rationalizing machines. We want an identity, a tribe. We'll give up anything for that.
It is horrifying and crushingly sad, often without any consolation.
[More upbeat posts coming. Sorry for the public wallowing. Song.]