* This is the text of a lecture given on 10 May 1965 at the Wellcome Historical Medical Library, London. It constitutes an abridged version of a lecture given in Paris in 1963 and published under the title –Casimir Davaine et les débuts de la Bactériologie médicale’, Confs Palais Découv., 1964, No. 95., pp. 32. The author wishes to thank Miss J. Lawson, Dr. J. Schiller, and Dr. F. N. L. Poynter, who have helped him in his translation.