Diphtheria antitoxin. Inoculated horse having blood taken from its exterior jugular vein, which will be used to treat diphtheria. Diphtheria is an acute infectious disease caused by the bacterium Corynebacterium diptheriae, which produces toxins that damage nerves and heart muscle. In 1890, Emil von Behring discovered that horses inoculated with C. diptheriae produced antitoxins (antibodies). Administration of horse blood serum to people with diphtheria neutralised the bacterial toxins and cleared the infection. Today, use of horse-derived antitoxins is less common, as most children are immunised against the disease.