INTRODUCTORY AND GENERAL 33 



substances which they produce are without deleterious effect on 

 the cells of the body. In another group of cases, referred to 

 above, the bacteria do not die, but their toxins remain harmless 

 to the host ; this is Ehrlich's immunitas non stevilisans, and it occurs 

 in the case of many of the lower animals which have in their blood 

 various protozoa (trypanosomes, etc.), without thereby suffering 

 the slightest appreciable injury. In man the condition is best 

 seen in its acquired form in the immunity possessed by negroes 

 to the action of malaria parasites, though the plasmodium may be 

 found in the blood. A closely allied phenomenon is in the latency 

 of bacteria. Thus a person may develop an attack of typhoid 

 osteitis years after an attack of typhoid fever, and we can only 

 assume that the bacteria have lain latent in his tissues for this 

 time ; in all probability they have been kept from infecting him as 

 a result of a sufficient degree of immunity, and when this breaks 

 down or wears off a renewed outburst occurs. The gonococcus 

 may be latent in a similar way for periods equally long. Another 

 similar phenomenon is the carriage of infection by persons who 

 remain themselves healthy. Diphtheria is a common example, 

 and it is no rarity to find a person in whose throat diphtheria 

 bacilli are present, but who remains unattacked. Here the 

 immunity suffices to prevent the bacillus from invading the body, 

 but not to destroy it. 



At the opposite end of the scale occur those cases in which 

 immunity is practically absent. Here the result of the introduc- 

 tion of the bacteria is a rapid infection, both local and general, 

 with profound symptoms of intoxication ; the bacteria spread 

 through the tissues just as they would through a good culture 

 medium, and, in addition, invade the blood and multiply therein. 

 This is rarely seen in man, though some examples of septicaemic 

 plague and streptococcal septicaemia from post-mortem wounds 

 approach it closely. It can be produced experimentally in 

 animals, when large doses of virulent cultures are injected. 

 Death follows in a few hours, and the blood is found to be swarm- 

 ing with bacteria. 



Between these two extremes come those cases in which the 

 introduction of the bacterium is followed by the production of a 

 local lesion. This always indicates some degree of local immunity, 

 and may be regarded as an attempt to localize the organism and 

 prevent its further spread. And the nature and severity of the 

 local lesion stand in close relation to the severity of the infection 



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