Robert Koch. XXill 
inquiries materially enhanced our knowledge of the causation and means of 
spread of these diseases. 
Koch’s work upon malaria needs special mention. Whilst in tropical 
countries, his attention was naturally drawn to this disease. lLaveran’s 
discovery of the malarial parasite had been made, but the mechanism of the 
spread of the disease was unknown. Manson’s discovery that filaria was 
inoculated by the mosquito, Theobald Smith’s proof that Texas fever was 
transmitted by ticks, and Bruce’s demonstration that the tsetse-fly disease was 
due to a protozoan parasite, and merely conveyed by the fly, suggested to Koch, 
as to others, that malaria might be transmitted by a biting insect. He was, 
indeed, engaged upon experiments with mosquitos, 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 harbour the parasite, 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 
years in conjunction with Schutz, upon the pathogenicity of the human 
tubercle bacillus for domestic animals. Briefly stated, Koch’s main con- 
clusion from their experiments was that human tuberculosis differs from 
bovine, and cannot be transmitted to cattle. The far more important 
question: “ Is man susceptible to bovine tuberculosis ?” was then considered. 
No direct experimental proof of this converse proposition is possible, but from 
the fact that men—and particularly children—consume large quantities of 
bovine tubercle bacilli in milk, and yet tuberculosis of the intestine is rare, 
Koch concluded that man is little, if at all, susceptible to the bovine variety 
of the bacillus. He pointed out that the question whether man is susceptible 
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 expen- 
diture with a view to minimising the chances of infection by milk and meat. 
Koch may have been unwise in stating his views, but he did so with the con- 
viction that bovine tubercle is not an important source of infection, and with 
the earnest desire that we should not squander our energies in subordinate 
directions, 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 inquiry in 
Europe and America. Of these the work of the English Royal Commission 
has been the most extensive. These investigations have shown that the 
