296 Professor J. Burdon Sanderson [May 15, 



by which infectious diseases are projDagated is called contagium^ a 

 word which may be conveniently used, provided that it is not 

 allowed to carry any suggestion that the disease to which it is 

 applied spreads by personal contact or intercourse. Like other 

 scientific terms, its use is to serve as a label for certain knowledge. 

 Under the heading contagium, the pathologist says (1) that all 

 contagia consist of organised (not merely organic) matter ; (2) that 

 this matter must, in order to be disseminated, be in a state of fine 

 division (particulate) ; (3) that the particles of which it consists 

 are living ; (4) that they derive their life (not as having been 

 themselves bits of the living substance of the diseased man or 

 animal, but) from parents like themselves. With reference to all of 

 these 2:)ropositions, excepting the last, there is agreement of opinion. 

 It is now eighteen years since it was proved by the investigations 

 of Chauveau that all the best known contagia (which are liquids of 

 the character of vaccine lymph) owe their activity to the minute, 

 almost ultra-microscopical, particles which float in them ; and no 

 one doubts that these particles are organised, and that their power 

 of producing disease depends on their organisation. Further, we 

 know, with reference to one or two diseases — namely, wcolsorters' 

 disease, or splenic fever, tuberculosis, leprosy, and one form of 

 septicaemia, that the particles in question are not only organised, 

 but themselves organisms — i. e. living individuals deriving their 

 life from parents like themselves. But from the moment that the 

 pathologist begins to infer that because in these particular instances, 

 which can be experimentally investigated, infection occurs by 

 organisms, it must be so in the case, for example, of cholera, of 

 which the behaviour is very different indeed from that of any of 

 the infectious diseases above enumerated, he leaves certainty behind 

 him and passes into the region of more or less probable conjecture. 

 With reference to the special question which now interests us, he 

 has to compare the mode of operation by which cholera spreads 

 with the modes of operation of those diseases which are propagated 

 by self- multiplying contagia — first, with a view to the estimation 

 of the antecedent probability that they are essentially identical ; 

 and secondly, to the testing of the estimate arrived at by such 

 experimental investigations as circumstances place within his reach. 



The antecedent probabilities may be stated as follows: — If the 

 reader will approach the subject with a mind freed for the moment 

 from metaphysical considerations, he will see that the spread of 

 cholera over the world must be due either to the dispersion of 

 infected persons, or of things with which such persons have been 

 in contact, or to the dissemination through the air of what may be 

 called " cholera-dust." The question whether there is such a thing 

 as cholera-dust rests on the teaching of experience as to whether 

 cholera can or cannot jump from one place to another at a distance 

 without the aid of personal intercourse. If this does occur it can 

 only be by dust. — i. e. minute particles of infective material sus- 



