PATHOGENIC MICROBES— INFECTION 123 



The vehicle of transmission may be a living creature instead ot 

 an inert object ; fleas carry plague from rat to rat and from 

 man to man. The intermediate inoculating agent may con- 

 stitute a storehouse and even a culture chamber for the virus, 

 e.g., the tick in spirillum fever. This intermediate agent 

 becomes properly speaking a host when the germ undergoes 

 in it, and can only undergo in it, a cycle of changes by which 

 it attains the stage at which it can infect us ; for example, the 

 mosquito for the parasite of malaria and the tse-tse fly for the 

 trypanosome of sleeping-sickness. The transmission of bovine 

 piroplasmosis from ox to ox is conducted by two individuals : 

 a tick becomes infected, produces larvse, and these larvse 

 inoculate another ox. 



Latent microbism is said to exist when the organs and tissues 

 contain germs which remain for a longer or shorter time 

 unsuspected. 



Pasteur thought that our organs and tissues were normally 

 aseptic : " The human body is completely closed to the intro- 

 duction of the germs of fermentation " ; and he was still more 

 right in adding, " Except the alimentary canal and again except 

 in certain pathological conditions.'' The body defends itself 

 well against the microbes which enter it ; " latent " microbes 

 can only be those which escape phagocytosis at least temporarily. 

 The best example is furnished by cases of spontaneous tetanus, 

 appearing under the influence of a heatstroke ; this tetanus is 

 due, according to Vincent, to the germination of spores which 

 have penetrated the body by some unknown path, and have 

 remained there for several days or even several weeks, till the 

 day when the excessive heat accompanied by fatigue interrupted 

 the phagocytic defence.' The experiments of Porcher and 

 Desoubry teach us that the blood is rarely aseptic during 

 digestion. 



Auto-infection is said to exist when an individual is infected 



^ Tetanus spores inoculated in the blood or under the skin of rabbits are 

 only eliminated after three or four weeks (subcutaneous inoculation), or 

 even after three months (intravenous inoculation). They 'wake up' and 

 germinate, should favourable conditions supervene, among others necrosis 

 in the tissues (Tarozzi). 



