104 



reason why it produces nausea, let us say, only, and not 

 fever, is because it is not strong enough to overcome the 

 vital force of the person attacked. Undoubtedly there are 

 various degrees of virulence. Apart even from Pasteur's 

 wonderful "vaccine" experiments we know that there are 

 mild and severe fevers, various degrees of diarrhoea, of which 

 Asiatic cholera (according to Jules Guerin and Sir William 

 Hunter) may be the most virulent form, and various degrees 

 of small-pox. In the Board of Health reports on the cholera 

 epidemic of 1848-9 it is stated that distinct warning of its 

 approach was given in every European city by the prevalence 

 of intermittent fever, dysentery, and especially diarrhoea; 

 and reports on subsequent epidemics have so fully confirmed 

 this observation that it may be taken as an axiom that 

 cholera is always preceded by epidemic diarrhoea. To a cer- 

 tain extent we may therefore provisionally regard the nature 

 of the gases in which microbes find themselves, in compari- 

 son with what I will call the standard of pure air, as deter- 

 mining the degree of parasitic virulence. In a paper read 

 at the meeting of the British Association at Southampton, I 

 ventured to suggest that the development of the tubercle 

 bacillus as a deadly parasite might be due, so to speak, to 

 its imprisonment in lungs inefiiciently aerated, either in 

 consequence of hereditary structure inducing weak breathing 

 habits, the habitual breathing of air containing less than the 

 healthy proportion of oxygen, or the choking of the air 

 passages through catarrh or the inhaling of dust. Miquel 

 points out that the proportion of "young" microbes is ex- 

 ceptionally large in sewers, where old or exhausted microbes 

 are rare. 



1.3. — But if we assume degrees of virulence from harmless 

 to deadly in one and the same species, we imply a 

 gradation from saprophyte to parasite, and from aerobic to 

 anaerobic. For even if attenuation by means of free oxygen 

 be regarded as the slow killing of the parasite, we can 



