86 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



as scientific evidence : a great many other people buy and eat 

 just the same mussels, but they do not take enteric fever. 



It is quite easy to make an explanation of this apparent 

 contradiction. Probably infection by organisms setting up 

 typhoid, and other infectious and contagious diseases, is far 

 more common than used to be imagined. Many of these 

 organisms are ubiquitous, and modern conditions of life must, 

 in many cases, greatly increase the chances of their distribution. 

 In no case do men and women yield easily to infection for the 

 defences set up by the normal healthy body are fairly strong. 

 The infection may not " take " at all (and pathologists must 

 encounter such failures, even in experimental work), and if 

 it does " take," it may successfully be resisted. There are 

 many ways by which Bacillus typhosus may be distributed — 

 by contaminated water, milk, vegetables and fruit, flies, carriers, 

 shellfish, personal infection, and perhaps also fried plaice. 

 Certainly some of these may be ruled out in many cases — water 

 and milk in modern conditions of public health administration, 

 for instances, but, as a rule, there must generally be more than 

 one means. Further, it is probable that there are conditions 

 which are necessary in order that the infection may take. 

 It is probable that the bodily " soil " must be such that the 

 pathogenic micro-organisms may grow : there may have to be 

 symbiosis with some other organism ; or a condition of 

 " rundownness " due to malnutrition, overcrowding, insufficient 

 warmth or clothing, etc. ; or some set of environmental con- 

 ditions which we do not understand. The progress of epidemics 

 does suggest this : that a number of conditions must coincide 

 and co-operate in order that the pathogenic organism (which 

 is thus only the immediate " cause ") may be enabled to operate 

 upon the bodily " soil." Thus public health practice, while 

 it may not neglect these exciting, or immediate causes, may 

 neither afford to neglect the essential co-operating ones. In 

 short, the role of shellfish as a contributory cause of disease 



