410 HUMAN BIOLOGY 



a cooperation of the blood plasma, the circulating nucleated 

 cells and certain fixed cells largely represented by units 

 of the reticulo-endotheial system, so that even when con- 

 siderable amounts of bacteria which cannot by themselves 

 gain entrance are experimentally injected into such animals, 

 they are promptly disposed of by a process not identical with, 

 but similar to that which is active in acquired immunity. 



Many possible balances in the reaction between the in- 

 vader and the host are conceivable, and for this reason it is 

 not surprising that in the natural evolution of infectious 

 disease many different types of relationship have been estab- 

 hshed. Thus, when hemolytic streptococci, pneumococci, 

 anthrax, plague or typhoid baciUi, or many other bacteria, 

 invade the body, they set up a violent reaction — an expres- 

 sion partly of their own toxic properties, partly of the ener- 

 getic effort of the host to get rid of them — and these together 

 constitute disease. In other cases such as the treponema 

 pallidum of syphilis, the organism, probably because it has 

 for centuries, been directly passed from body to body, 

 without intermediate external existence, is so perfectly 

 adapted to the tissues of man that it creates little acute 

 disturbance. Injury is manifest only after considerable 

 accumulation, and we have a chronic and slowly destructive 

 disease. This state of affairs approaches the quasi sym- 

 biotic conditions observed in certain sarcosporidial and 

 spirochetal diseases in mice and in trypanosome infections 

 in rats, in which it may be said that infection has developed 

 into an adaptation so perfect that the host no longer reacts, 

 and manifest disease does not follow. At opposite ends on the 

 series, then, we may have disease without infection, as in 

 diphtheria and tetanus, where the bacteria do not invade 

 but manufacture externally a toxin which is absorbed; and 

 infection without disease, as in the last cases mentioned. 



When bacteria that are capable of causing a fatal infection 

 in an animal or in a human being enter the body, immediately 

 a struggle is set up in which the bacteria grow and elaborate 

 any poisonous substances which they are capable of produc- 

 ing. Both b}^ their presence in the intercellular spaces and 

 by the toxic inflammatory effects of their constituents and 

 products, they injure the cells of the immediate neighbour- 



