Emile Roux preparing croup vaccine. 1889 illustration of the French physician, bacteriologist and immunologist Emile Roux (1853-1933) preparing a vaccine. Croup, a respiratory disorder, was frequently caused by diphtheria. It was often fatal before the use of diphtheria vaccination. Roux worked with Louis Pasteur, and was a co-founder of the Pasteur Institute in 1887. His 1888 publication on the cause of diphtheria identified the bacterium responsible, and he successfully developed a serum to treat it.