Quackery is derived from the word quacksalver, which historically was an insult used originally against heroic physicians like Paracelsus. It was historically associated with the use of calomel / mercury in Europe as medicine. The German form of the word is quacksalber,and is based on the word quecksilber in German which means quicksilver or mercury.
Who where the Quacks who originally tropically treat diseases like syphilis with salves made from Mercury?
It was the educated and then politically correct professional physician, of course.
"Quackery used to be a pejorative term describing medical charlatanism. As medical charlatanism became more popular and as using pejorative terms became politically incorrect except for the formerly oppressed classes, quackery evolved into alternative medicine and complementary medicine by those who practice it, and into unproven therapies and questionable methods by those who are critical of it. When quackery is mixed with scientific medicine, the latter is called a mainstream modality and the result is called integrative medicine by those who practice."
Quackery was yesterdays alternative medicine, is today's complementary medicine, and will be tomorrow's new branch of medicine. What is considered Quackery is purely a matter of politics.
A classical example would be physical medicine or physical therapy. Historically, physical medicine clearly developed at the turn of the twentieth century from Osteopathy and Chiropractic; which were originally considered forms of Quackery because they offered alternative methods of treatment that competed with medicine. Physical therapy developed as a response to this alternative treatment competition to the point where conventional medicine now falsely claims to have invented physical medicine directly from science.