DISTRIBUTION OF PATHOGENIC BACTERIA llo 
the particular group, and quite distinct from methods of prevention and 
control of microorganisms belonging to the other three groups. 
This simplifies and unifies the problem of the epidemiologist; it is 
unnecessary for example to enter into a separate detailed account of the 
epidemiology of typhoid and of dysentery and of cholera, because the 
points of escape from the host, the paths through the environment, back 
to the host again are the same for each microbe. Thus soil, food, milk 
and shell fish alike may become infected with feces or urine containing 
any or all of the causative agents of the excrementitious diseases and 
thus become directly or indirectly the vehicles through which the 
microbes are brought back again to successive, prospective hosts. 
Infected droplets, or utensils infected with the microorganisms of the 
respiratory diseases, pass directly or indirectly to the respiratory tracts 
of healthy persons in exactly the same manner even though the bac- 
teria or carriers of this great group of diseases are separate entities. 
The menace of the venereal patient, both socially and epidemiolog- 
ically is the same whether he be a syphilitic or a case of gonorrhea. The 
details of treatment are of course unlike but the communal and epidem- 
iological aspects are the same. 
With respect to insect-borne diseases involving an extrinsic vector, 
a knowledge of the biology of each particular insect is essential for the 
control of the disease. To this extent, the epidemiology of this group 
of infections is more complex than that of the respiratory, intestinal 
and venereal diseases, where man and microbe alone are involved. 
One additional factor deserves mention: as the progressively patho- 
genic bacteria are viewed from the standpoint of these four several 
principal epidemiological types it becomes evident that a number of 
important details, which collectively comprise the biology of disease- 
producing organisms, are missing. Thus, the great question of seasonal 
and annual occurrence of particular kinds of infections, manifested by 
respiratory and intestinal infectants of man particularly, is unsolved. 
Also, the causes of the maturation and recession of epidemics, the vary- 
ing severity and decline of specific diseases still remain a puzzle, in spite 
of the vast amount of excellent investigation that has been spent upon 
them. 
The study of the natural history of disease, the very essence of pre- 
ventive medicine, may be confidently expected to reveal new relation- 
ships of importance which are not as yet dreamed of. A suggestion of 
the unexpected gain which such studies may reveal may be apprehended 
through that remarkable diminution in respiratory diseases which not 
unfrequently has accompanied the installation of adequate water 
filters, which presumably ])rotect a community against excrementitious 
diseases only. This phenomenon, known as the Mills-Reincke ])he- 
nomenon, has never been satisfactorily explained. It is readily 
comprehended that water filters will reduce the incidence of typhoid 
or other contagious intestinal infections, but it is difficult to understand 
in the light of present knowledge, just how a concomitant reduction in 
diseases not water-borne should occur. 
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