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 decrease, 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 sometimes 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 communicable 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, however, 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,
