In 1796, while translating a treatise by the Scottish physician William Cullen, the German doctor Samuel Hahnemann rejected Cullen’s claim that cinchona bark cured malaria because it was bitter. He swallowed the bark himself, developed fever, chills, and aching joints, and concluded that a drug cures a disease because it causes similar symptoms in a healthy person. This episode traces how that single self experiment, made by a physician who had quit practice in disgust at bloodletting and mercury purges, grew into homeopathy: the law of similars, the provings in which volunteers logged every symptom, and the Materia Medica Pura of 1810.

Raw doses made patients worse, so Hahnemann diluted them and struck the vials against a leather bound book. A 30C remedy is diluted by a factor of 100 thirty times over. He later blamed most chronic disease on invisible miasms, chiefly psora, the itch. By 1900 the United States had 22 homeopathic colleges and more than 100 homeopathic hospitals, whose cholera patients died at lower rates, the episode argues, because they were spared bleeding and calomel. Modern meta analyses find the remedies perform no better than placebo.

• Quinine, not the law of similars, is why cinchona bark works against the malaria parasite
• Hahnemann’s provers were barred from spices, wine, coffee, tea, and even chess, which he thought too exciting
• The episode’s scale for a 30C pill: to hold a single atom of the original substance it would need to be as wide as the distance from Earth to the Sun
• Why he believed treating a skin rash pushed disease deeper into the body
• The UK’s National Health Service and health authorities in Australia, France, and Spain have moved to defund or ban homeopathy