Croup vaccination. Artwork of doctors and nurses giving a vaccination against croup to a child in a hospital ward. At far right is Emile Roux (1853- 1933), French bacteriologist. Croup is an infect- ion of the upper airways which occurs mainly in children up to the age of four. It causes a hoarse voice, a barking cough and inflammation of the airways. Croup can be caused by viral or bact- erial infection, spasm, allergy or inhalation of a foreign body. One disease which can cause croup is diphtheria. Roux developed an anti-toxin serum for diphtheria which was first used on patients in 1894, halving the sufferers' mortality rate. Image of the Trousseau Hospital, France, by Brouillet.