THE ORIGINS OF ZYMOTIC DISEASES 179 



communication were poor and beset by dangers. A journey 

 from York to London was then a more serious affair than a 

 journey from London to San Francisco is to-day. As in 

 Pacific islands, water- and especially air-borne diseases were, 

 therefore, absent during long periods of time. When they 

 came they spread as epidemics. Accordingly we read of 

 plague and pestilence ; of diseases suddenly becoming epi- 

 demic and sweeping away a fourth or a half of entire com- 

 munities. Historians are apt to attribute these immense 

 catastrophes partly to the bad sanitation of the period and 

 partly to diseases which have died out of the world, or at 

 any rate out of Europe. Doubtless they are right in a few 

 instances. But, apart from diseases which spread under 

 special circumstances from tropical centres, bad sanitation, 

 under modern conditions of intercommunication and crowd- 

 ing, tends to render water-borne disease endemic, not epidemic. 

 Over air-borne disease it has no effect. Measles, whooping- 

 cough, chicken-pox, influenza, common cold, and small-pox (in 

 a modified form) are as common as ever. 1 The character of 

 these ancient epidemics, their special symptoms as indicated 

 in old literature, their sudden and portentous appearance 



1 The foregoing has an interesting bearing on the vaccination contro- 

 versy. It is abundantly evident that sanitation has not banished small- 

 pox. We owe much to modern sanitation. It has immensely reduced 

 the water-borne diseases. It has to some extent diminished the earth- 

 borne diseases. But it has failed totally against air-borne diseases. It is 

 possible to keep untainted our water supply, and to clean and disinfect 

 our nouses. But we can neither keep untainted nor disinfect the air. 

 Its volume is too vast, its flow too swift. A great many people contend 

 that vaccination has not banished small-pox. Let us for a moment, and 

 for the sake of argument, grant that contention. It follows, since im- 

 munity against small-pox is acquired, not inborn, and since so few of us 

 have suffered from the disease, that practically the whole community is 

 susceptible. Under these circumstances what has caused the almost 

 complete disappearance of a disease which until lately was endemic and 

 almost universal among us 1 We are told isolation. The answer indi- 

 cates surprising confusion of thought. We cannot isolate any one when 

 every one is susceptible at any rate, in the case of air-borne disease. 

 The attendants of the sick themselves fall ill and spread the infection. 

 Isolation is possible only when the great mass of the community is im- 

 mune, when only the exceptional individual is capable of taking the 

 disease. Under opposite conditions the pestilence spreads like a prairie 

 fire. By itself isolation has no greater power of controlling small-pox 

 than the historic old lady with a broom had of sweeping back the 

 Atlantic. In the absence of vaccination it would be worse than useless. 

 Better it were a thousand times that small-pox should be endemic 

 amongst us than it should be epidemic. In the former case many 

 people would lose their lives, and practically the whole community 

 would be disfigured. In the latter case the very existence of the race 

 would be menaced. 



