AIR AND EARTH-BORNE DISEASES 453 



of vaccination ; that is, by means of an artificially attenuated form 

 of each disease. By such measures smallpox, the most dreaded of 

 all air-borne maladies, has been almost banished from various parts 

 of the civilized world, and would have quite disappeared from the 

 globe had the whole human species for the term of a generation 

 been vaccinated at intervals of a few years. Thereafter, the 

 virulent microbes, having become extinct, vaccination itself would 

 have become unnecessary. This conclusion may appear ex- 

 travagant ; but, if we take all the facts into account, it is difficult 

 to escape from it. In Germany, for example, smallpox seems 

 never to appear except as an importation from abroad. What 

 is possible in one country is possible in others. 



742. Whether internal sanitation can be made as effective a 

 weapon against air-borne disease in general as it is against 

 smallpox in particular can only be known in the future. As yet 

 we have made no more than a beginning. It may be useful 

 only against such diseases as usually confer permanent immunity, 

 and perhaps it will ultimately prove impossible to attenuate many 

 air-borne diseases in the way smallpox has been modified that 

 is, to bring about such an attenuation in the nature of the microbes, 

 that, unable to exist in the blood-stream, they are confined to a 

 single locality in the body, and so are incapable of reaching the 

 air passages and infecting the atmosphere. Antitoxins, since 

 they induce only a ' passive ' or temporary immunity, cannot serve 

 the same purpose. Moreover, in order to manufacture antitoxins, 

 it is necessary to keep alive the virulent microbes. 



743. Immunity to leprosy and tuberculosis, the principal earth- 

 borne diseases, cannot be acquired by the individual. Therefore, 

 internal sanitation supplies no weapon against them. The 

 infectivity of leprosy, however, is very low. Owing apparently to 

 the isolation of sufferers, and the increase of personal cleanliness, it 

 has been banished from most civilized communities. There can 

 be little doubt that ultimately it will disappear altogether. On 

 the other hand, tuberculosis, principally a disease of the air passages, 

 is, relatively, highly infectious. 1 External sanitation improve- 



1 1 have termed tuberculosis an earth-borne disease, because its bacilli rest 

 when outside the body mainly on the floor, walls, and furniture of dwellings. 

 But since they are inhaled with floating dust it is also in a real sense an air-borne 

 malady. To me the evidence that it enters the body in the vast majority of cases 

 through the lungs appears overwhelming. But many medical men, notably 

 Behring, favour the hypothesis that the alimentary canal is the main portal 

 of infection, and that the milk of tuberculous cows is the principal vehicle. It is 

 supposed that bacilli, imbibed during infancy, " remain latent until they find 



