In 17th-century Netherlands, snake-oil sellers were called kwakzalvers. That’s why today we call them "quacks." Regardless of the name, they all do the same thing: sell useless and often harmful medical treatments and health advice to a defenseless public. Some of them may well believe the claims they are making, and sincerely think they’ve hit upon a remedy or a treatment approach that no one else has thought of before but that works. Others simply don’t care whether what they’re hawking does anything or not. Regardless, the results for the patient are the same—no improvement and, often, a delay in seeking or using remedies that might actually improve their health.
That is the most consistent danger of quackery. People will spend time, money, and other resources on treatments that don’t treat anything. Even if these fake treatments are harmless in themselves, the illness isn’t being treated—and may be getting worse—while the patient goes on this wild goose chase. Sometimes, however, the treatment is actively harmful. Colloidal silver turns the skin blue. "Black salve," a concoction sold to skin cancer sufferers, can be incredibly damaging, eating away at the skin it is supposed to heal. Worst of all, some hucksters convince perfectly healthy people that they need a lifestyle change to preserve their health, or suffer an ailment that has no symptoms and may well be unknown to medical science, which they need to pay the quack to cure.
There are some indicators that strongly suggest that a proposed treatment is nonsense. One is universality—a medication that treats a broad range of very different ailments may sound like a wonder drug, but the truth is that isn’t generally how drugs work. A real medication is generally specifically aimed at a particular cause or part of the body. Some quacks offer an alternative hypothesis about disease, medicine, or even anatomy, claiming it is superior to the conventional medical understanding. At the same time, many will use scientific-sounding language and other external trappings of legitimate medicine. Quacks will often claim that the medical establishment is trying to silence them or keep their discoveries secret, including by branding them quacks. The word "toxins" is a key sign, especially when it is not further elaborated upon.