MAN AND THE MICROBE 9 



day's work and would starve if left to his own resources, so the microbic 

 parasites in adapting themselves to live at the expense of their human 

 host have lost the capacity to gain a living in the world outside. They 

 have been so modified in the course of evolution as to thrive in the rich 

 warm fluids of the body and perish outside of it. If a hundred typhoid 

 germs are discharged into a lake their fate is much the same as that of a 

 hundred men under the same conditions. A few of the men may be good 

 swimmers and a few may be lucky enough to cling to floating planks. 

 Most die very quickly, however, and in the course of time all will surely 

 perish. So with most disease germs in water or soil or anywhere outside 

 the body. Certain pathogenic microbes may actually multiply in suitable 

 media, in milk for example. As a rule, however, there is a steady de- 

 crease, rapid at first and slower afterward, but inevitably leading to 

 extinction in a comparatively short period. We read of disease germs 

 persisting in dust or ice for several months, and a very few may some- 

 times do so. Quantitative studies show, however, that the survivors are 

 few indeed and that the danger from such remote infection is practically 

 negligible. In the case of water, which has been more carefully studied 

 than any other medium, we know, for example, that in a period of two or 

 three weeks even gross infection will be removed by the natural mortality 

 of the microbes. Dr. Houston, of London, who has done some of the 

 most important work upon this subject, has repeatedly demonstrated his 

 confidence in his results by drinking halfpint portions of water, merely 

 kept in bottles for a few weeks in the laboratory after infection with 

 millions of typhoid bacilli. Both bacteriological and epidemiological 

 evidence indicates very clearly that it is only fresh, recent infectious 

 material which plays an important part in the transmission of com- 

 municable disease. 



This conclusion is one of the most important fruits of recent sanitary 

 research; for it focuses attention sharply upon the human being, the 

 original source of virulent disease germs, rather than upon vague and 

 obscure miasms of the earth and air. It is people, primarily, and not 

 things that we must guard against. Certain media, like milk and water, 

 are important agents in the transfer of infection from person to person. 

 Others, like air and dust and fomites (books, toys and the like which 

 have been exposed to infection) are known to be far less dangerous than 

 was supposed. Back of all such material agents of transmission, how- 

 ever, lies the human being, and the nearer to this source we get, — the 

 more direct and rapid the transfer, — the greater is the danger. 



Unfortunately, however, it is not only the obviously sick person 

 who may be a center for the distribution of active disease. Another of 

 the great contributions to sanitary science in the last ten years has been 

 the recognition of the part played by incipient cases and "carriers," 

 apparently well persons, who are nevertheless discharging from nose and 

 throat or bowels the virulent germs of disease. Measles, for example, 



