Homeopathy is a 200-year-old therapeutic system that uses small doses of various substances to stimulate autoregulatory and self-healing processes. Homeopathy selects substances by matching a patient's symptoms with symptoms produced by these substances in healthy individuals. Medicines are prepared by serial dilution and shaking, which proponents claim imprints information into water. Although many conventional physicians find such notions mplausible, homeopathy had a prominent place in 19th-century health care and has recently undergone a worldwide revival.