SMALLPOX 239 



in the horse it becomes horse-pox ; transferred back to man 

 it is vaccinia. By then the nature of the microbes has been 

 profoundly altered. Owing probably to a loss of virulence 

 (i.e. offensive and defensive power), they are unable to spread over 

 the body, but are restricted to the spot of inoculation, where they 

 and their toxins are most concentrated. The toxins, however, 

 permeate the body, and induce a general change, from which 

 results recovery from vaccinia and immunity to smallpox. The 

 absence of the microbes from the blood-serum, and therefore from 

 the lungs, prevents their diffusion by the breath. Probably it is 

 for this reason that vaccinia, unlike smallpox, is not air-borne. 

 Consequently, it does not infect people in the vicinity. It has 

 become like rabies and syphilis, a disease which is communicated 

 only by direct contact and under particular circumstances. 1 It 

 does not, during its passage through a series of human hosts, again 

 evolve the ancient characteristics of smallpox because, on the one 

 hand, the failure of its microbes to pass naturally from a sufferer 

 to his fellows results in the extermination of all that are not 

 artificially removed, so that there is no survival of the fittest as re- 

 gards the power to migrate, while, on the other hand, man artificially 

 enables the more and the less fit to migrate alike. Human races 

 which have had no ancestral experience of smallpox, which have 

 not been weeded out by it, have comparatively low powers of 

 resistance. A party of Esquimaux, who visited Berlin and were 

 vaccinated there, developed a general disease resembling or identi- 

 cal with smallpox, and perished of it. 2 The microbes, therefore, 

 were able to spread over the entire body. Doubtless if vaccinia 

 were passed through a series of such people, and then through 

 more resistant types, it would again become smallpox just as 

 harmless saprophytic organisms become virulent parasites if passed, 

 under favourable circumstances, through a series of living hosts. 



401. (g) Rabies in dogs is a very deadly disease. If passed 

 through a series of rabbits, it becomes still more virulent, at any rate, 

 for rabbits. If passed through a series of monkeys it becomes milder. 

 Pasteur secured complete immunity from virulent rabies by inocu- 

 lation first with the mildest type (from monkeys), then with 

 more virulent types, and lastly, with the most virulent type (from 

 rabbits). Subsequently he discovered an even better method, that 



1 Not all ' general ' diseases, malaria for example, are air-borne. To be air- 

 borne it is necessary not only that the microbes shall find their way to the air 

 passages, but also that they shall be adapted for transmission through the air. 



2 The Scottish Medical and Surgical Journal, April 1900, p. 330. 



