150 APPLIED BACTERIOLOGY 



in a large tank, and passed through a centrifugal separator, 

 whereby the cream is collected. Thus, it is evident that if 

 the milk of one farmer was contaminated with typhoid, it 

 would be the means of the conveyance of the disease over 

 a large area. The only method of prevention of the spread 

 of infection by the contamination of milk would be proper 

 sterilisation, which would have to be systematically carried 

 out all the year round. 



An epidemic of typhoid fever due to the milk-supply will 

 exhibit some or all of the following features : (1) The out- 

 break is sudden, and many of the attacks are simultaneous. 

 (2) A large proportion of the households attacked have a 

 •common milk-supply. (3) The incidence of the disease 

 will be greatest on the principal consumers. 



Conveyance by Vegetation. — Enteric fever has been known 

 tc be sonveyed by vegetables grown on sewage farms, and 

 also by watercress grown in sewage-polluted streams. 



Conveyance by Shell-fish. — Oysters, mussels, etc., which 

 have come from water contaminated by drainage, may be 

 a source of infection. Poote has recently been carrying out 

 a series of interesting investigations on the vitality of 

 typhoid bacilli when inoculated into oysters. For the first 

 fourteen days after introduction, the typhoid bacilli multi- 

 plied, but after some time a steady decline in the number 

 of microbes took place. Thirty days after the bacilli were 

 first introduced into the oyster their presence was still 

 demonstrable, they having been preserved in the stomach of 

 the oyster, where they retained their vitality unimpaired. 



In some experiments the water containing the oyster was 

 infected with the bacilli, and it was found that they actually 

 lived longer in the body of the oyster than they did in the 

 water containing the latter, which seems to distinctly point 

 to the possibility of contracting typhoid through the con- 

 sumption of the bivalve. 



