440 INFECTION AND IMMUNITY. 



perature curve. This condition may be discovered 

 by blood cultures. It is thought that the strepto- 

 coccus does not increase the toxicity of the typhoid 

 bacillus, the result being rather a summation of 

 the intoxication of the two infections. Post- 

 typhoidal suppurations are often due to the strep- 

 tococcus and in many of the metastatic complica- 

 tions (parotitis, pleurisy, peritonitis, meningitis, 

 otitis media) streptococci and staphylococci have 

 been found. Pneumococcus pneumonia not infre- 

 quently complicates typhoid fever. A combined 

 infection of typhoid and malaria is said to occur in 

 the tropics; the complication is grave. Typhoid 

 and diphtheria may occur together, and typhoid 

 may be superimposed on acute tuberculosis. 

 immunity The period of greatest susceptibility to typhoid 

 ld t s ib s mty: is found from the fifteenth to the twenty-fifth 

 years. The resistance of infants and children is 

 not satisfactorily explained. A certain amount of 

 resistance inherited from the mother may persist 

 for some years after birth. It is known that anti- 

 bodies may pass from the mother to the fetus 

 through the placenta. In very early life the tissues 

 may respond more energetically to incipient in- 

 fection by the rapid formation of typhoid anti- 

 bodies, or the phagocytic cells may be more active. 

 The conditions which render older people less sus- 

 ceptible are no better understood. A loss of suit- 

 able receptors may have occurred so that the toxic 

 constituents of the bacilli find no anchorage in the 

 body, or the affinity between the receptors and the 

 toxic constituents may have become less. The in- 

 dividual during the course of years may have been 

 gradually immunized by the entrance of non- 

 pathogenic quantities of the bacilli into the cir- 



