PROCEEDINGS OF SECTION I. 589 



upon consideration of the sewage disposal system, especially in 

 relation to flies. Virtually, therefore, there are two great pos- 

 sibilities — firstly, direct excretal pollution of food, milk, water, 

 such as occurs in the excreta-water, excreta-milk, excreta-food- 

 stuff, items of infection; and, secondly, fly pollution, which prac- 

 tically must be an excreta-fly-food or excreta-fly-milk outbreak. 



In the second or excreta-fly group, the means of infection is 

 fairly obvious. With the first group, however, the initial con- 

 tamination of the water, milk, or food-stuff may be very hard to 

 trace. With water and milk the problem is perhaps easier than 

 with food, for the latter, especially if it be shellfish or ice cream, 

 has often quite a long history before the time it is made up and 

 sold to the consumer, and opportunities for infection may occur 

 in very many points in its commercial history. With all means 

 of infection of typhoid the long incubation period often hides the 

 primary infecting agent, but this is all the more so with certain 

 varieties of food, and thus it is, although such epidemics must 

 occur, we so seldom hear of a proved case of cold meat, brawn, 

 or cold pudding infection. The food stuff has gone a fortnight 

 before the case develops — there is no chance of sheeting home the 

 bacteriological proof. 



In the case of repeated infection that occurs, say in the case 

 of a carrier milk infection, the actual demonstration of the bacilli 

 in the milk may be possible, and in any case the chain of evidence 

 will lead in many cases fairly directly to the carrier probably em- 

 ployed in some part of the process of milk preparation or dis- 

 tribution. Similarly in the case of water infection, although 

 the actual bacilli giving rise to the first sei'ies of cases will not 

 be often recoverable, still in certain cases the source of infection, 

 unfiltered sewage or excreta, may give rise to further infections 

 of the water during one of which periods tlie bacilli may be re- 

 coverable. I should like to point out here how a rarely occurring 

 chain of circumstances may be required for the spread of infec- 

 tion from a certain source, and that although the source may be 

 all the time present the infection from it may only occur at com- 

 paratively rare intervals. Thus a chronic carrier working in a 

 dairy may only prove infectious under certain exceptional circum- 

 stances. His hands ma)', if he is reasonably clean, only rarely 

 be contaminated from fseces or urine, and again it may only be 

 a rarely occurring circumstance that will enable him to inoculate 

 the milk. For instance, he may only rarely be employed as a 

 milker, or infection may occur on some isolated occasion when 

 perhaps he has removed a fly, &c., from the milk with his finger^ 

 Again it is well to remember that typhoid fever most likely re- 

 quires a massive infection, and that most likely only when cir- 

 cumstances allow multiplication of the bacilli after the accidental 



