The 18th century continued to be marked by unsupported theories. The German physician and chemist Georg Ernst Stahl believed that the soul is the vital principle and that it controls organic development; in contrast, the German physician Friedrich Hoffmann considered the body a machine and life a mechanical process. These opposing theories of the vitalists and the mechanists were influential in 18th-century medicine. The British physician William Cullen attributed disease to the excess or deficiency of nervous energy; and the physician John Brown of Edinburgh taught that disease was caused by weakness or inadequate stimulation of the organism. According to his theories, known as the Brunonian system, stimulation should be increased by treatment with irritants and large dosages of drugs. In opposition to this system, the German physician Samuel Hahnemann developed the system of homeopathy late in the 18th century, which emphasized small dosages of drugs to cure disease.

Other unusual medical practices developed toward the end of the 18th century include phrenology, a theory formulated by the German physician Franz Joseph Gall, who believed that examination of the skull of an individual would reveal information about mental functions. The theory of animal magnetism developed by the Austrian physician Franz Mesmer was based on the existence of a magnetic force having a powerful influence on the human body.

Despite these unorthodox medical practices, the end of the 18th century was marked by many true medical innovations. British physicians William Smellie and William Hunter made advances in obstetrics that established this field as a separate branch of medicine. The British social reformer John Howard furthered humane treatment for hospital patients and prison inmates throughout Europe. In 1796 British physician Edward Jenner introduced vaccination to prevent smallpox. His efforts both controlled this dreaded disease and also established the science of immunization.