Anything related to health care in general, really. Keep in mind that germ theory was only invented in the late 16th century, and it was ridiculed for centuries in favour of Miasma theory. It wasn't until the mid 19th century that it started gaining legitimacy.
Ancient wisdom often sounds like common sense now that it is commomly taught. What is some ancient wisdom that we no longer teach because it was wrong?
This question inspired by this post..