[fooker]

The most prominent historical example is Semmelweis, dude wanted doctors to start washing hands. Was ridiculed, bullied, and abused into insanity.

I'll leave out mrna vaccines and gender related examples to avoid politically charged examples and people missing the forest for the trees.

Here are some more.

https://en.wikipedia.org/wiki/John_Abramson (Read his book!)

https://en.wikipedia.org/wiki/Nancy_Olivieri

https://en.wikipedia.org/wiki/David_Healy_(psychiatrist)

https://en.wikipedia.org/wiki/Frances_Oldham_Kelsey

You can google the general topic and see that there are hundreds more, but none of these individuals managed to become prominent or well known because they just gave up.