There are only people who think they understand the body because they don't know what a p-value means. As indicated by the replication crisis, where over 50% of results are not even repeatable, the majority of this info is incorrect. And that is the tip of the iceberg because even if an experiment/observation is consistent that does not mean you interpreted the results correctly.

Actually, I bet there are very few people who actually have a better understanding of the human body than me, just because I have the appropriate skepticism.