274 EPIDEMIC AND ENDEMIC DISEASE 



disease, and no other disease of this kind has been so affected. It 

 cannot have been isolation, for, if vaccinia is no protection, almost 

 the whole community is susceptible, and doctors and nurses should 

 contract and spread the disease. Isolation is possible only when 

 susceptible individuals are comparatively rare. It has been denied, 

 even, that there is such a thing as acquired immunity against any 

 disease. But the fact that the microbes of acute disease, which at 

 first swarm in an infected person, disappear within a definite time, 

 so that the sufferer no longer conveys infection after recovery, is 

 decisive proof. The fact that a vaccinated person cannot be made 

 to contract vaccinia again soon after recovery is proof that it con- 

 fers immunity against itself. The fact that he can be successfully 

 vaccinated after an interval of years is proof that the immunity is 

 not perpetual. The fact that smallpox, if experimentally trans- 

 ferred to the calf, becomes cow-pox, and if transferred back to man 

 becomes vaccinia, is proof that the latter is a modified form of 

 smallpox. The fact that recently vaccinated persons are able, 

 with almost complete safety, to nurse people ill of smallpox, 

 whereas unvaccinated persons are almost sure to contract the 

 disease, is proof that experience of the attenuated malady affords 

 protection from the more virulent type. The fact that in England 

 most people who contract smallpox have been vaccinated, but not 

 recently, is proof that the protection afforded by vaccinia is only 

 transient so transient, owing to the mildness of the disease, as to 

 be midway between 'acute' and 'passive' immunity. Probably 

 the agitation against vaccination would never have arisen in this 

 country had the power of conferring permanent immunity from 

 smallpox not been claimed for vaccinia by the medical men of 

 a past generation. Reaction followed the discovery that most 

 sufferers from smallpox had undergone vaccination. The public 

 learns very slowly, but forgets no faster. 



455. Water-borne diseases are almost as infective as the air- 

 borne type. They prevail as epidemics more rarely in isolated 

 communities, but as such are not uncommon in densely populated 

 centres. The microbes are very dependent on the environment 

 outside the body of the host, therefore they are less able to 

 travel than air-borne species, and seasonable changes may for 

 a time purify the water-supply, even where they are most 

 prevalent. 



456. Earth-borne diseases, of which the only one that greatly 

 affects human beings is tuberculosis, are always endemic. Direct 

 sunlight is fatal to the microbes of tuberculosis. In effect, it can- 



