THE EPIDEMIOLOGY OF TYPHOID FEVER 381 



are naturally insusceptible to typhoid fever or who have 

 developed immunity in consequence of a previous attack ; they 

 may constitute a danger to susceptible persons with whom they 

 may come in contact. The most serious danger to a community 

 arises, however, from the " chronic " carrier. In certain carriers, 

 the focus of multiplication of the typhoid bacillus may not be 

 the bowel but the kidney or bladder, the bacilli in such cases 

 passing out in the urine. 



The tracking down of a typhoid carrier constitutes an impor- 

 tant and difficult problem. Firstly, the serum of all suspicious 

 persons ought to be subjected to the Widal test (vide infra). 

 Usually speaking, the carrier gives a positive reaction, but some- 

 times this is absent and sometimes is only obtained with a low 

 dilution of the serum. Further, it has been shown in chronic 

 carriers that the agglutinating capacity of the serum varies from 

 time to time and sometimes may be absent. The proof of the 

 presence of a carrier lies essentially in the isolation of the 

 typhoid bacillus from the faeces or the urine, and it is to be 

 noted that, especially in the former, the organism is not con- 

 stantly present, in certain cases months of remission have been 

 recorded. Several explanations have been advanced to account 

 for the facts observed, such as the occurrence of symptomless 

 reinfections or of periodic more or less acute auto-infections from 

 a latent focus of persistence of the bacterium in, e.g., the gall- 

 bladder. In any case, the necessity for repeated investigation of 

 a suspected carrier is obvious. The methods to be adopted are 

 detailed on p. 389. Much work has been directed to the question 

 of freeing the typhoid carrier from the organism, but although 

 various methods, such as intestinal antisepsis, vaccination, excision 

 of the gall-bladder, ^have been tried, success has hitherto not been 

 attained. From the public health standpoint, the prevention of 

 carriers from occurring in a population must be provided for, 

 and in fever hospitals means ought to be taken for retaining 

 convalescents from typhoid until the bodily discharges are free 

 from the typhoid bacillus. This has already been undertaken 

 in the British army in India. 



The Epidemiology of Typhoid Fever. In civilised com- 

 munities the prevalence of typhoid fever has been very markedly 

 reduced, coincident with the substitution of central filtered water 

 supplies for well waters and with the improvements effected in 

 general sanitation and especially in the rapid removal of refuse. 

 In certain localities, however, there are still periodic outbreaks, 

 often of a seasonal character, and it has been customary to 

 attribute these largely to the capacity of the typhoid bacillus 



