From The Greatest Benefit to Mankind: A Medical History of Humanity, by Roy Porter:

Writing this book has not only made me more aware than usual of my own ignorance; it has brought home the collective and largely irremediable ignorance of historians about the medical history of mankind. Perhaps the most celebrated physician ever is Hippocrates yet we know literally nothing about him. Neither do we know anything concrete about most of the medical encounters there have ever been. The historical record is like the night sky: we see a few stars and group them into mythic constellations. But what is chiefly visible is the darkness.