DISTRIBUTION OF PARASITIC AND PATHOGENIC BACTERIA 89 



longer or shorter time, these tubercles eventually break down, typically 

 into the air passages and discharge there large numbers of tubercle 

 bacilli. These are coughed up by the patient and are eliminated from 

 the body, usually in enormous numbers, by droplets and in the sputum. 

 Pulmonary tuberculosis is typically a chronic, focal disease. The 

 perpetuation of the tubercle bacillus is assured through their elimina- 

 tion from the diseased body in enormous numbers through long periods 

 of time, their ability to resist desiccation, and the relative directness 

 with which they reach other hosts. 



The typhoid bacillus gains entrance to the body through the mouth 

 and the intestinal tract. The organisms penetrate the intestinal 

 mucosa, develop in the internal organs, particularly the spleen, and 

 after a rather definite excursion in the tissues of the* body, enter into 

 the intestinal tract again, either through ulcers or the gall-bladder or, 

 occasionally, they appear in the urine. They are eliminated from the 

 body in great numbers, either with the feces, or less commonly, the 

 urine, and they gain access immediately to other subjects through 

 direct contact or more or less indirectly through water or food, in 

 sufficient numbers to set up infection in at least some of them. 



The gonococcus is transmitted directly by contact. Occasionally 

 the infection may be somewhat less direct, involving the conjunctiva. 



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 temporarily, 

 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, which acts potentially as an hypodermic syringe, as it were, 

 in this instance. 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 mechanism of escape from one V)st 

 to the other. 



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 



