French physician and bacteriologist who made important contributions to the diagnosis, treatment, and prevention of many diseases.

In 1896 Widal developed a procedure for diagnosing typhoid fever based on the fact that antibodies in the blood of an infected individual cause the bacteria to bind together into clumps (the Widal reaction). A professor of pathology and internal medicine at the University of Paris (1911–29), he also recognized (1906) the body's retention of sodium chloride (salt) as a feature of nephritis (inflammation of the kidney) and cardiac edema (accumulation of excessive fluid in tissues as a result of heart disease), recommending salt deprivation in the treatment of both diseases. He demonstrated the increased fragility of red blood cells in cases of hemolytic jaundice and, with the French physician Georges Hayem, described the acquired form of the disease (the Hayem–Widal type, 1907). During World War I, Widal prepared an antityphoid–paratyphoid vaccine that appreciably reduced typhoid contagion among the allied armies.