INFLUENCE OF DISEASE UPON MILK 75 
greatly in different localities and in different herds 
in the same section. The proportion of animals reacting 
to the tuberculin test ranges from none in herds which 
have been subjected to annual tests for several years to 
30 per cent. and over in herds in which no effort has been 
made to control the disease. 
2. Virulence for Man of Tubercle Bacilli from Cattle.— 
—Until 1901 it was very generally accepted that tuber- 
culosis in man and animals was the same disease, although 
Theobold Smith, in 1896, and, subsequently, others, 
pointed out important differences in virulence, morphol- 
ogy, and cultural characteristics between bacilli from 
human and bovine sources. In 1901 Koch announced 
that tuberculosis of cattle was so rarely transmitted to 
man that it could practically be disregarded in formulat- 
ing plans to protect man against the disease. This an- 
nouncement was based on the failure of Koch and Schiitz 
to infect calves and other animals with tuberculous ma- 
terial from man, and upon post-mortem statistics col- 
lected by them of a number of cases of tuberculosis in man 
which happened to include only a small proportion of in- 
dividuals showing primary lesions in the digestive tract 
or attached lymph glands. Koch’s announcement made 
a pronounced impression upon the general public, al- 
though his experiments were not original nor were his re- 
sults undisputed. ‘Theobold Smith, Frothingham, and 
Dinwiddie in this country, and Piitz, Gaiser, Nocard, 
McFadyean, Thomasson, Chauveau, Klebbs, Kitt, Bol- 
linger, and Crookshank abroad, had previously at- 
tempted to infect cattle with tuberculous material from 
man and had succeeded in doing so, although they found 
that these animals were less susceptible to human tuber- 
culous material than to that from bovine sources. Since 
