In every community in which typhoid fever is endemic, the desire exists for a method of determining the presence or absence of immunity to the disease.
A test that could be relied on to furnish an index of resistance to invasion by the typhoid bacillus, would determine the need of prophylactic vaccination or revaccination, and in certain less common instances, perhaps afford the means of identifying a previous obscure infection as typhoid fever. The shortcomings of the ordinary immunological procedures for this purpose, long recognized, were emphasized anew by Gay and his co-workers, who devised the typhoidin reaction to supplement them.
Gay and Force1 prepared an extract of the typhoid bacillus according to the technic employed by Koch in the preparation of "Original Tuberculin ;" 250 c.c. of 5 per cent. glycerin broth were inoculated with a strain of Bacillus typhosus (Dorset Army Strain No. 5), and