Ignaz Semmelweis was a Hungarian physician living in the 19th century. The medical university of Budapest is named after him, and for a good reason: he discovered by 1847 that the leading cause of childbed mortality was the fact that doctors at the time did not wash their hands between patients.
In those days, autopsies were performed on everyone, and doctors switched between them and attending to pregnancies, which ended up killing children.
Despite the fact that he could prove this through statistics of hospitals where his method was applied, child deaths dropped, he died in an insane asylum some days after the guards beat him up.
The medical community wouldn't accept his discoveries, and it took almost a generation until the methodology became widespread and Louis Pasteur established germ theory some twenty years later.
In the meantime doctors just kept killing kids.
The Semmelweis effect or reflex is the phrase used to describe this phenomena, when new knowledge or new evidence is rejected outright as it contradicts established dogma.
One such established dogma today is that we have moved passed these kind of errors. We're 21st century people with 21st century science and technology, so surely we won't succumb to such fallacies any more right?
Let me turn that question upside down.
How many things do you know about that are surrounded by loads of popular and stubborn misconceptions?
It's pretty clear that our society is still suffering from Semmelweis effects in many areas. There's a replication crisis in science to start, public opinion is continuously being manipulated by savvy spin doctors as how something is perceived socially forms a large part of why it's considered valuable (or not).
And we can also see that many debates are now basically tribal, with each side having their own "dogmatic view" of the issue as well as what counts as established evidence. And that this phenomenon is pervasive. I'm pretty sure I could google any random mundane subject and would immediately run into a rampant Semmelweis effect.
We might not be killing kids with our ignorance, but Semmelweis reflexes are still doing harm. Consider the people who are getting burned badly right now on the crypto market. Their perception of these coins as solid investments were manipulated so hard by con men, that many are still refusing to accept that they were scammed - and no evidence will convince them otherwise.
This is why it feels that arguments - especially on the internet - are pointless. Instead what we have is the rise and rise of mockery communities: essentially split factions of a given dogma who throw crap at those perceived to be still hypnotized.
And the terrifying thing about this is that we're all at risk of getting had one way or another by this. It's hard to keep an open mind especially on touchy subjects. In fact research shows that for example posting on social media actually hardens your resolve on the matter: sort of the same way as public proclamations work.
There's nothing guaranteeing that we won't again. As for the rest of the harms: nobody seems to be keeping tabs on those.