This chapter tabulates the seriousness of diphtheria as a cause of national mortality relating to England. It provides the maximum and minimum death rates between 1871 and 1921 for diphtheria and a number of other acute infectious diseases. The discovery of antitoxin for diphtheria was a triumph of laboratory research, the outcome of experimental work on immunity. The progress of this research can be seen from stage to stage, and it constitutes an epochal point in the history of medicine. By Schick's test it is possible to distinguish persons liable to be attacked by diphtheria and to immunize or otherwise protect these, while ignoring others whose Schick reaction is negative. Recently diphtheria toxin has been treated with formalin, and its toxicity thus reduced, and it would appear that use of toxoid and toxoid-antitoxin gives good results. The method has also proved its value in protecting nurses exposed to diphtheria infection.