MODE OF SPREAD OF INFECTIVE DISEASES. 769 



depend on whether the germs can live in the place to 

 which they are transported, whether they find sufficient 

 nutrient materials to permit their growth, &c. 



Thus it is only seldom that we are able to pick out 

 the true mode of transport from the many possible ones. 

 This want of knowledge, and our open acknowledgment 

 of it, does not, however, render it more difficult to take 

 prophylactic measures against a typhoid epidemic. In 

 whatever way the spores have been transported they are 

 in all probability introduced into the body by water and 

 food ; and they only leave the body by the dejecta. If 

 the latter are thoroughly disinfected or removed by a 

 good system of drainage, if the water is taken from 

 proper wells, and if the selection and preparation of 

 food is attended to, a typhoid epidemic can be satis- 

 factorily coped with even though the exact mode of 

 transport in each individual case remains unknown. 

 The closure of suspected wells is also a perfectly proper 

 procedure, though the reason for doing so is not now 

 the same as formerly; the former practice led to in- 

 sufficient and probably erroneous views as to the etio- 

 logical importance of the water, and to too great one- 

 sidedness in the prophylactic measures. 



It is evident that the great multiplicity of the factors 

 which play a part in the local and seasonal distribution 

 of all infective diseases makes it extremely difficult to 

 draw conclusions as to the nature and the infective 

 properties of the causal agents from these local and 

 seasonal variations and from the laws which appear to 

 govern them. At the present time, when we can study Difficulties in 

 the behaviour of the infective agents themselves, and eddmAtiDgtte 

 can solve the question of their mode of distribution local and 

 experimentally, it would help us but little to employ the disposition' " 

 former circuitous method by which we drew conclusions 

 as to the characters of the infective agents from the 

 phenomena of their local and seasonal distribution; it 

 is much better to attempt to gain a solid basis from 

 experiments with the isolated infective agents them- 

 selves, and thus to obtain an explanation of the results 

 of epidemiological statistics. 



49 



