1 6 INFECTION AND IMMUNITY 



fever patients the bacillus may be carried by the wind, 

 with dust, and deposited upon the moist mucous 

 membrane of the mouths and nasal passages of indi- 

 viduals who breathe this dust-laden air. This is also 

 true of the bacillus of bubonic plague, and to a still 

 greater degree of the bacillus of tuberculosis. 



The bubonic-plague bacillus, contained in the ex- 

 creta of infected individuals and of rats, which are 

 very susceptible to the disease, may retain its vitality 

 for a considerable time when deposited upon the 

 ground, and it is in this way that insanitary localities 

 become centres of infection. The tubercle bacillus, 

 which is contained in the matter coughed up from 

 the lungs by persons suffering from pulmonary con- 

 sumption, may retain its vitality and infecting power 

 for a long time after the expectorated material con- 

 taining it has been dried and pulverised. 



It is evident that such dust is likely to be carried 

 by currents of air and deposited in the lungs of per- 

 sons who are compelled to live in localities where 

 such insanitary conditions prevail. No doubt this is 

 the usual way in which pulmonary consumption is 

 contracted. Again, the dust deposited in the mouth 

 and nasal passages may be swallowed and other forms 

 of tubercular disease result tubercular peritonitis, 

 tubercular meningitis, tubercular joint disease, tuber- 

 culosis of the vertebral column. 



