HOW PARASITIC AND PATHOGENIC BACTERIA REACH MAN 89 
The plague bacillus may be transmitted from host to host, either 
directly in the case of pneumonic plague, where great numbers of 
plague bacilli are coughed up from the lungs of one patient and trans- 
mitted through inhalation to other patients, or somewhat more 
indirectly, as is the case in bubonic plague. Bubonic plague appears 
to be a true septicemia; the plague bacilli circulate, at least tempo- 
rarily, in the blood, and they are removed from the blood of one 
patient and transmitted to another patient (either man or rat) through 
the agency of the flea. When the flea bites it defecates, and plague 
bacilli are excreted with the feces. The irritation caused by the bite 
leads the victim to scratch, and in so doing, plague bacilli are rubbed 
through the skin, causing infection. Plague bacilli are locked up in 
the tissues of the host, and were it not for the agency of a suctorial 
insect, as the flea, bubonic plague would almost certainly disappear, 
because the organisms have not perfected for themselves any mechan- 
ism of escape from one host to the other, except in the pneumonic type 
of the disease.^ 
IV. DISTRIBUTION OF PARASITIC AND PATHOGENIC BACTERIA 
IN NATURE. 
It has been shown in previous sections that comparatively few, if 
indeed any, of those bacteria habitually parasitic or pathogenic for 
man are found in Nature far removed from rather intimate associa- 
tion with their hosts. This is in accordance with the fact that few, 
if any, of these organisms are provided with spores which would 
enable them to survive exposure to long periods of conditions unfav- 
orable to their growth. It is true, however, that some, at least, of 
these organisms, as for example, the typhoid bacillus, can survive for 
longer or shorter periods of time in the soil, particularly if it be frozen, 
or in water, for days or even weeks. There is little evidence that 
these bacteria multiply extensively outside the body; on the contrary, 
they tend to die off rather rapidly. In any event, their existence 
depends upon their reaching a suitable host again within a compara- 
tively brief period. 
There are a few spore-forming bacteria which occasionally infect 
man when associated conditions are favorable for them. They are 
found in the intestinal contents frequently, from whence they escape 
to the soil. Of these the bacillus of lockjaw, B. tetani; of botulism, 
B. botulinus; the gas bacillus, B. aerogenes capsulatus; and the anthrax 
bacillus, B. anthracis, are well known. 
V. HOW PARASITIC AND PATHOGENIC BACTERIA REACH MAN. 
A. The Occurrence of Parasitic Bacteria Upon the Bodies of Healthy 
Men and Animals.— The continual exposure of the skin of man to his 
1 For a discussion of the relations between Parasitism and the Metchnikoff Theory of 
Immunity and Pathogenism and the Ehrlich Theory of Immunity, see page 132. 
