446 MICRO-ORGANISMS AND DISEASE [chap. 



brought about, the inference, it must be admitted, that the 

 cholera vibrio is the microbe of cholera is extremely near. 

 It cannot be expected that in a number of healthy persons 

 the ingestion of laboratory cultures, which, as experiments 

 on the guinea-pig show, are liable to become less and less 

 virulent, should be productive of severe and typical attacks 

 of cholera ; even at the beginning and towards the end of 

 an epidemic of cholera we see occasionally a considerable per- 

 centage of mild attacks — practically only more or less severe 

 diarrhoea ; therefore, that in the above experiments there 

 should have been a percentage of positive results — cases with 

 fairly severe diarrhoea — and in the one case of Metchnikoff's 

 series a severe result should have been actually brought about, 

 and that in all these positive cases the comma baciUi introduced 

 should have multiphed in the intestine and their presence 

 been demonstrated by microscopic and culture test, is 

 in itself a very strong link in the evidence as to the causative 

 relation between the vibrio and the disease cholera. 



Another important link of evidence was brought forward 

 by showing that the blood-serum of a person who had 

 recovered from an attack of Asiatic cholera possesses the 

 power to confer passive immunity to guinea-pigs against the 

 action of the cholera vibrio (Klemperer, Botkin, Wasser- 

 mann), that is to say that serum possesses immunising action 

 against the cholera vibrio. If the cholera vibrio is really 

 the cause of cholera, one can understand that, just as in 

 other communicable diseases a first attack alters or adds 

 something to the blood, so as to furnish this with immunising 

 power, a hke effect should be produced by the cholera 

 vibrio after it has been growing and multiplying within the 

 affected individual, that is to say that the blood-serum of 

 such an individual should possess a specific immunising 

 action against the cholera vibrio. And this is actually the 



