CHAPTER VIII. 



BACTERIA CAUSING CHRONIC INFECTIONS. 



THE BACILLUS OF TUBERCULOSIS. 



TUBERCULOSIS is an infectious disease caused by 

 the tubercle bacillus, which was discovered by Profes- 

 sor Koch in 1882. The organism is widely dis- 

 tributed over the world, and is pathogenic for the 

 lower animals as well as for man. It is frequently 

 found in cattle, less often in goats and swine, rarely in 

 sheep, horses, dogs, and cats. 



The bacillus is a slender rod, slightly curved, with 

 rounded ends. It is purely parasitic, that is, it will not 

 grow or multiply outside a host. It is never found 

 save in the bodies and discharges of animals affected 

 by the disease, or in the dust or upon articles which 

 the discharges have contaminated. It is not motile, 

 does not form spores, and is cultivated on artificial 

 culture media with difficulty. It cannot grow with- 

 out a liberal supply of oxygen, and only at body tem- 

 perature. It is killed by moist heat at 70 C. in ten 

 minutes, but dry heat at 100 C. requires one hour. 

 Direct sunlight destroys them in two hours, but when 

 protected from sunlight they can live for a year. 



There are four kinds of tubercle bacilli: the 

 human; the bovine, chiefly found in cattle; the avian, 

 found in birds, and the reptilian. The human tubercle 



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