A quack is defined by Google.com as "a person who dishonestly claims to have special knowledge and skill in some field, typically in medicine." There are other words relating to a quack, such as humbug, charlatan, con artist, and swindler. Overall, anything recommended by a quack will not be useful. In fact, it may be quite dangerous. It may just take your life.
Most of the time, quacks lured in their victims by promising medical miracles. The cures themselves are so dubious that only the desperate would try them. However, the authors of Quackery: A Brief History of the Worst Ways to Cure Everything suggest that quackery is not just about pure deception but, in fact, includes situations when people believe that what they're selling may actually be working. Some examples of common practices include the citizens of the Ottoman Empire eating clay to keep the plague away and the Victorians using mercury steam rooms to cure syphilis.