Sympathetic medicine is a form of traditional medicine in which people are treated with things which physically relate to their medical conditions. For example, sympathetic treatments for eye conditions would use eyes, in the theory that the power of the eye could be transferred to the patient, thereby curing the condition. Although sympathetic medicine is an important part of many folklore traditions, it should come as no surprise to learn that it is often not terribly effective.
According to the principles of sympathetic medicine, the physical appearance of objects in the natural world is meant to provide clues to their properties. Plants of all sorts have commonly been used in sympathetic medicine. In the case of a patient with joint pain, for instance, a practitioner might look for a plant with knobby, clubbed joints and use it in an herbal treatment which could be applied to the joints or ingested. The folklore of sympathetic medicine is quite complex, with practitioners passing down information about the efficacy of various plants and the best ways in which they could be used.
The concept of sympathetic medicine is not widely used in most medical traditions in the modern world, but it does linger on in some places. In Traditional Chinese Medicine, for example, some treatments have a sympathetic aspect, such as eating parts of animals which are traditionally thought of as virile to increase one's own virility. Some plants which have been used in sympathetic medicine for centuries continue to be used for the same purposes today, suggesting that in some cases, the physical appearance of something may indeed have an incidental relationship with its properties.
Many practitioners of sympathetic medicine have also integrated a magical component into their practice. It is not enough to merely use objects which relate to someone's illness; the practitioner must also weave spells and prayers over them to make them effective. This concept has been adopted in a variety of mystical traditions, such as voodoo, in which objects are infused with curses and cures through prayer and ritual.
As you might imagine, the historical practice of sympathetic medicine carried some dangers to patients. Some of the substances used in sympathetic medicine were toxic, for example, potentially putting the patient at risk of serious illness or death. The practice of smearing substances on wounds to heal them was also not terribly productive, because it could cause severe infections, especially when additives like dung were used.