110 SAPROPHYTISM, PARASITISM AND PATHOGENISM 
excited by it is focal, chronic and slow-going. Years may elapse 
before the host finally succumbs. The development of the organisms 
within the tissues of the host does not appear to lead to the formation 
of substances which arouse the latent offensive and defensive mech- 
anism of the host to acute antagonism. During this long period 
the tubercle bacilli establish communication with the outside world, 
and, in a majority of cases, countless myriads of bacilli escape from 
the host, affording ample opportunity for the infection of new hosts, 
before death removes him as a source of infection. Occasionally 
tubercle bacilli become widely disseminated in the body, causing rapidly 
fatal, generalized miliary tuberculosis. These organisms perish with 
their host being locked up in the body without means of escape. 
It is well known that the virulence of bacteria, many kinds at 
least, can be increased decidedly by passage from animal to animal 
by providing an artificial portal of entry and of exit from animal to 
animal. This is accomplished by injecting the organisms into a 
first animal and reinjecting them, at brief intervals, into other animals. 
In such instances there is a direct continuity of growth from animal 
to animal, greater than is met with in naturally occurring infections. 
It is worthy of note that bacteria of the "opportunist" type are, gen- 
erally speaking, more successfully exalted in virulence under these 
conditions than the progressively pathogenic forms. 
There is yet another feature of pathogenism which is worthy of 
note. From time to time almost any bacterial disease, for example, 
pneumonic plague or influenza, may leap suddenly to epidemic pro- 
portions, spread rapidly and then subside again, to be succeeded by 
sporadic car,es which gradually diminish in numbers and in severity. 
It is significant that these great pandemic outbreaks are frequently 
of the type of droplet infections. The bacteria causing these outbreaks 
appear to acquire somehow and somewhere, an unusual degree of 
invasiveness and they spread rapidly, especially in thickly settled 
areas and along the great arteries of communication, and as rapidly 
lose their unusual activities and subside to what appears to be their 
usual level of virulence. It is very probable that those strains of 
pathogenic bacteria in general, which suddenly acquire unusual 
^'irulence are short-lived in epidemic areas, partly because their hosts 
perish before the microbes can escape to new hosts. Not infre- 
quently, these or similar epidemics are preceded by mild, atypical 
diseases, which may not be specifically recognized, and during this 
initial period the bacteria may be quite widely distributed. ^ 
VII. SEASONAL AND ANNUAL DISTRIBUTION OF PATHOGENIC 
BACTERIA. 
Epidemiologists have long recognized that certain types of infection, 
especially those associated with the respiratory and intestinal tracts, 
1 Kendall: Boston Med. and Surg. Jour., 1915, 172, 851. 
