From the Latin bos (“ox” or “cow”) Mycobacterium bovis is a virulent bacterial species originally isolated from tubercules in cattle. Robert Koch, who discovered the tubercle bacillus in 1882, believed that M. bovis was not a danger to humans. Theobald Smith and others established beyond doubt that, contrary to Koch’s belief, M. bovis could infect humans but was not the usual source of human infection. In 1908, French scientists Albert Calmette and Camille Guérin chose an M. bovis strain for their work on a tuberculosis vaccine. They repeatedly subcultured the isolate on a mixture of glycerol, potato, and bile for 13 years until it was sufficiently attenuated to be used as a vaccine. The bacillus Calmette-Guérin (BCG) vaccine was adopted by the League of Nations as the standard tuberculosis vaccine in 1928 and continues to be used in most developing countries.