The classic example of quackery is homeopathic medicine. The basis of homeopathy is that if a substance causes a disease, extremely small doses of that substance can stimulate the body to fight against the disease. In this way the person does not contract the disease. The process of immunization is well understood, and the principles of homeopathy do not fall into this category. The substance used to treat an individual is not the actual cause of the disease, but rather a chemical which, if ingested by a healthy individual, results in the same symptoms of the disease.
Many diseases have multiple symptoms, with no two individuals being exactly alike.
Therefore, what would work in one individual, would not necessarily work in another, even though they had the same disease. If homeopathy doesn't work, how can cures using homeopathic medicine be explained. First is the placebo effect. If you give people a chemical that they believe will cure them, many times they will be cured. It has nothing to do with the chemical but rather the influence of the individual's psychology.