148 TUBERCULOSIS 
tuberculosis. To what extent man becomes infected from the bovine 
variety cannot be stated, but the accumulating evidence tends to the 
conclusion that bovine tuberculosis is of less significance in its 
influence upon public health than was formerly thought, and of more 
importance as a rapidly spreading and destructive disease among 
cattle. Concerning its transmission, the conclusion seems to be 
warranted, that the virus of tuberculosis spreads very largely among 
men and cattle from individual to individual of the same species 
rather than from one species to the other. Swine are often infected 
from cattle. 
Geographical distribution. Tuberculosis is an exceedingly wide- 
spread disease. In earlier times it was quite prevalent among cattle 
in Central Europe. It seems to have existed in Western Asia and 
Northern Africa at an early date. From these centers it has spread 
to nearly every cattle raising country of the world. Its rapid spread 
during the last fifty years is attributed to the increase in cattle 
exchange resulting in the introduction of tuberculous animals into 
healthy herds. It is stated that in many countries, and in large dis- 
tricts within others, tuberculosis did not exist until it was introduced 
within recent years by the importation of diseased animals. 
In countries where there has been little or no importation of cattle, * 
and in which the native breeds still exist unchanged, as in many parts 
of Russia, Austria and Spain, in the northern part of Sweden and 
Norway, and in parts of Africa, tuberculosis is practically unknown. 
This is true of the cattle on the island of Jersey, where for more than a 
hundred years foreign cattle have not been introduced. 
In the United States, the disease is very widely distributed. It is 
found to a considerable extent in certain localities where the climatic 
conditions seem to be beneficial for tuberculous people. The explana- 
tion for this seems to be that tuberculous animals have been intro- 
duced into these districts. There are, however, large areas in which 
it is practically unknown. The Western steers that are killed in the 
large slaughter houses are practically free from this disease except 
those that come from a few infected regions. 
Etiology. Tuberculosis is caused by a rod-shaped organism known 
as Bacterium tuberculosis. It was discovered by Robert Koch in 1882. 
Schiller and Toussaint had previously studied growths which seem, 
from the results of their inoculation experiments, to have been due 
to this organism. The bacterium of tuberculosis is a slender, rod- 
