INFECTION 119 
understood and reduce his natural resistance to microbic invasion on the 
one hand, and his ahihty to rally his specific, anti-microbic mechanism 
on the other hand. Intercurrent disease frequently weakens the 
initial lines of defense, permitting bacteria of the "opportunist" type 
to become invasive. Thus, furunculosis frequently is a complication 
of diabetes, pneumonia not uncommonly terminates a case of tuber- 
culosis. Renal and cardiac disease may weaken the normal barriers 
of the body, permitting a variety of infections with endogenous 
bacteria. 
It is a well-attested fact that certain occupations or professions 
cause or promote pathological conditions which predispose to infec- 
tion. Prominent among these is participation in those arts or indus- 
tries which involve exposure to poisonous or irritating dust or fumes. 
The incidence of tuberculosis among those frequently exposed to 
organic or inorganic dusts is a striking example of the relation of 
occupation to infection. 
When an exogenous invading microorganism has reached a suitable 
atrium of the body,^ overcome the initial defense of the host at that 
point, and has successfully resisted the normal humoral or cellular 
opposition of the host, it is free to grow in the tissues without opposition. 
All of the defenses of the host have been overcome. Then a new phase 
of the struggle becomes prominent, during which the host gradually 
develops a specific attack upon the invader, bringing into action latent 
forces which, developing as specific oftensive weapons, or antibodies, 
eventually rid the body of the invader in favorable cases. The host 
is usually immune to subsequent invasion by the same organism. The 
invader also may change its weapons in some degree to meet the anti- 
microbic activity of the host,- and the result of the struggle may be 
complete recovery from infection, chronic disease, the bacillus carrier 
state, or death of the host. 
The initial and secondary defensive powers of the host, therefore, 
are both cellular and humoral in character. The intact skin and 
mucous membranes of the gastro-intestinal, respiratory and genito- 
urinary tracts are important initial non-specific lines of defense. The 
phagocytic activity of leukocytes and certain fixed tissue cells, and 
the natural, normal bactericidal substances of the blood and lymph, 
which bathe the initial line of defense, are important non-specific 
adjuvants in maintaining the integrity of these initial barriers to infec- 
tion. In certain infections the humoral factors are the more important, 
while in others the cellular mechanism is conspicuous. 
The defensive mechanism against the same bacterium may be 
different in one or another animal. For example, dogs and rats are 
relatively immune to infection with the anthrax bacillus. The immu- 
1 It should be emphasized that pathogenic bacteria enter the tissues of the body at 
certain locations which are distinctive for each separate kind of pathogenic microbe. 
On the other hand, parasitic bacteria, unable of themselves to penetrate to underlying 
tissues may enter the tissues at any point where a passage is opened for them. 
2 See Welch: The Huxley Lecture, British Med. Jour., 1902, ii, 1105. 
