PATHOGENIC MICROBES— INFECTION 129 



cells. But the same disease may be in different phases 

 septicsemic or localised in a tissue, and in any case the blood 

 itself is simply a tissue whose cells are motile. It was long 

 thought that typhoid fever was an infection localized to the 

 small intestine and to the Peyer's patches there, but in reality 

 it is septicsemic during the whole of the febrile period. There 

 are also septicsemic phases in pneumonia and tuberculosis. 



Microbes are not inert particles but living cells acting 

 through their secretions and toxins. Cholera is an intestinal 

 infection, but it is fatal to its host from a general intoxication. 

 Every infection is to some extent an intoxication. Even the 

 macroscopic parasites, to which formerly only a physical 

 activity was ascribed, the bothriocephalus, the ankylostoma, 

 the trichina, secrete poisons which have been studied. 



Microbes, ferments, and toxins are inseparable terms, and 

 for this reason the discovery of the diphtheria toxin by Roux 

 and Yersin in 1888 began a new era in bacteriology. 



