MODE OF SPREAD OF INFECTIVE DISEASES. 767 



in the soil ; on the contrary, the whole series of factors 

 which influence the local and seasonal occurrence, and 

 which have been referred to above, must be taken into 

 consideration just as much as in the case of those affec- 

 tions which are only contagious and which are in no 

 wise influenced by the soil, but which nevertheless also 

 show local and seasonal variations (scarlet fever, small- 

 pox). Among these factors the chief are certain habits 

 of life, selection and preparation of food, water supply, 

 cleanliness of the person and of the dwelling, and the 

 previous occurrence of epidemics with the consequent 

 immunity. 



There are a number of factors which we can with Drinking 

 more or less certainty refer to as affording a certain 

 amount of explanation of the smaller epidemics 

 typhoid fever which often show a marked local and typhoid fever, 

 seasonal occurrence. At present there is a tendency 

 among physicians to find something remarkable and 

 only explicable by the assumption of a special and 

 specific mode of spread in almost all cases of typhoid 

 fever, whether they are limited to a single house, or 

 occur in neighbouring houses, or pass over streets ; the 

 usual explanation has been the presence of wells con- 

 taining so-called impure water. 



It is no doubt quite true that the typhoid bacilli are 

 carried by the water in many large and in numerous 

 small epidemics, but it is equally certain that in a large 

 number of cases this mode of transport has been assumed 

 without sufficient justification. The view is also 

 generally held that it is only by means of the drinking 

 water that we can explain those cases where a small 

 circle of people become affected around the small central 

 epidemic, some of the persons in this circle being passed 

 over (these usually not having partaken of the water), 

 and where in addition a few individuals outside this 

 immediate circle become affected, these being shown 

 to have also consumed this water. On examining such 

 cases, however, we very often find that the statistical 

 facts were insufficient and incorrect, because they only 

 take into account a small portion of the population 



