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 ioo° C. requires one hour. 
Direct sunlight destroys them in two hours, but when 
protected from 1 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 
(95) 
Morphol¬ 
ogy 
