Why medieval people didn't wash their hair, and how it stayed clean

Medieval people didn't bathe or wash their hair, right? Hair care in the middle ages involved no shampoo, but they still kept their hair clean. As a professional hairstylist, I think this historical myth needs to be debunked. Busting this myth comes down to understanding what resources Medieval people had for personal care, and reveals that historical people were actually quite clever about working with what knowledge they had.