Pierre Paul Emile Roux (1853-1933) was a French physician, bacteriologist and immunologist who was one of the closest collaborators of Louis Pasteur, a co-founder of the Pasteur Institute and discoverer of the anti-diphtheria serum, the first effective therapy for this disease. Roux dedicated himself to investigations on the microbiology and practical immunology of tetanus, tuberculosis, syphilis, and pneumonia.