Between 1908 and 1921, Albert Calmette and Camille Guérin cultured and attenuated Mycobacterium bovis through 239 successive passages until obtaining a strain that conferred immunity without causing disease. On 18 July 1921 the first dose was administered to a newborn in Paris. BCG is the first major anti-tuberculosis vaccine and a direct crystallization of two traditions: the Pasteurian principle of attenuation and Koch's causal bacteriology. More than a century after its introduction, it remains the vaccine with the most doses ever administered in the history of medicine, with more than four billion people vaccinated.