ROBERT KOCH. 657 
that malaria might be transmitted by a biting insect. He was, indeed, 
engaged upon experiments with mosquitoes, and had nearly satisfied 
himself that malaria was thus transmitted, when Ross published his 
results. Koch was, however, largely instrumental in showing that 
the three types of malaria were associated with three distinct parasites, 
and that none of these were infective for the lower animals, a result 
of great importance, from the point of view of malaria prophylaxis. 
He also cleared up the difficulty as to the reservoir of the disease in a 
population the adults of which could not be found to harbor the para- 
site by showing that the young children, even to the extent of 90 to 
100 per cent, were infected. 
In 1901 Koch reported to the British Congress on Tuberculosis the 
results of experiments which he had carried on during the preceding 
two tears in conjunction with Schutz upon the pathogenicity of the 
human tubercle bacillus for domestic animals. Briefly stated, Koch’s 
main conclusion from their experiments was that human tuberculosis 
differs from bovine and can not be transmitted to cattle. The far 
more important question: ‘Is man susceptible to bovine tubercu- 
losis ?’”’ was then considered. No direct experimental proof of this 
converse proposition is possible, but from the fact that men—and par- 
ticularly children—consume large quantities of bovine tubercle 
bacilli in milk, and yet tuberculosis of the intestine is rare, Koch con- 
cluded that man is little if at all susceptible to the bovine variety of 
the bacillus. He pointed out that the question whether man is sus- 
ceptible to bovine tuberculosis at all was not decided, but expressed 
the belief that infection of human beings is of so rare occurrence that 
it is not necessary to take any measures against it. It was the last 
conclusion that caused so much consternation, as most countries were 
embarked in considerable expenditure with a view to minimizing the 
chances of infection by milk and meat. Koch may have been unwise 
in stating his views, but he did so with the conviction that bovine 
tubercle is not an important source of infection, and with the earnest 
desire that we should not squander our energies in subordinate direc- 
tions, but should concentrate them in efforts to diminish man-to-man 
infection through the respiratory tract. 
The importance attached to a considered opinion of so distinguished 
an authority led to the appointment of numerous commissions of in- 
quiry in Europe and America. Of these the work of the English 
Royal Commission has been the most extensive. These investiga- 
tions have shown that the sharp distinction between the two varieties 
though usually manifest, is not so absolute as Koch supposed, and 
that bacilli of the bovine type are not so uncommonly found in human 
infections as he was led to believe. The frank expression of opinion 
by Koch on this subject has been the stimulus for an enormous 
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