112 SAPROPHYTISM, PARASITISM AND PATHOGENISM 
dents of the body. They do not ordinarily incite epidemic disease, 
and therefore they are of no great concern to the epidemiologist. 
The pathogenic bacteria^ on the contrary are alien to the body. 
They are the causative agents in contagious diseases. Their cycle of 
perpetuation involves typically the successful completion of a complex 
series of events during which they must reach a suitable host, directly 
or indirectly invade the tissues of that host, multiply therein, and in 
vastly increased numbers escape to other, susceptible hosts. 
It is convenient to divide the pathogenic bacteria into four rather 
distinct groups, with respect to their epidemiology. Each has its 
particular method of transmission, its characteristic portal of entry to 
the tissues, its method of escape from the tissues, and from the body 
of the host, and its distinctive symptomatology. 
The first, and in many ways, the simplest of these groups is exem- 
plified by the venereal diseases. Here the organisms are implanted 
directly into the atrium, and invasion usually occurs very promptly. 
The path of escape to other hosts is also equally direct in most instances, 
although it may be somewhat more circuitous, through the agency of 
towels, or some such means. 
The second type of transmission is through the medium of insects. 
The puncture, bite, or at least the agency of the insect carrier provides 
the requisite portal of entry for the microbe to the tissues, and also 
of its escape from the tissues. The bacteria that are thus carried habit- 
ually do not ordinarily exist free outside the host, and their destribu- 
tion corresponds quite definitely with that of their insect vector. 
The third group of pathogenic bacteria comprises the invaders of 
the tissues through the respiratory tract. They escape from the host 
chiefly in infected droplets of sputum, and frequently pass directly 
to successive hosts through the inhalation of these infected droplets, 
or somewhat less directly through infected utensils carried to the 
mouth. 
The fourth group of pathogenic bacteria comprises the intestinal 
infectants. The bacteria of this group must enter the mouth of the 
prospective host, run the gauntlet of the stomach, and pass to some spot 
in the intestines, as Peyer's patches, through which invasion may take 
place. After a period of growth in the tissues, the microbes, greatly 
increased in numbers, escape to the outside through the feces, less 
commonly the urine, and thence by food, fingers, flies, water or utensils 
reach other susceptible hosts. 
It is quite obvious from the foregoing discussion that the epidemi- 
ologist is confronted with four principal and quite distinct types of 
disease transmission, each including a group of microbes having a 
definite portal of entry to the host. Also, the methods for prevention 
and for control of contagious disease are equally divisable into four 
types, each of which is generally applicable to the bacteria comprising 
' The terms Parasitic and Pathogenic are variously employed in Zoology, Botany and 
Bacteriology, therefore they are defined here to indicate the manner in which they are 
to be construed in this discussion of Parasitism and Pathogenism. 
