I7O, BACTERIOLOGY. 



one attack of an infectious disease this chemical 

 substance is secreted, and, remaining in the system, 

 opposes the onset of the micro-organism at a future 

 time. In the natural immunity of certain species 

 and individuals we must suppose that this chemical 

 substance is normally present. 



Another theory is, that the micro-organisms 

 assimilate the elements which they require for their 

 nutrition from the blood and tissues, and render 

 the soil impoverished or otherwise unsuitable for 

 the development of the same micro-organisms here- 

 after ; this condition may be permanent, or the 

 chemical constitution of the tissues may be restored 

 to normal, when immunity ceases. If. however, 

 we explain acquired immunity by the result of the 

 growth of a previous invasion of micro-organisms, 

 we are still confronted with the difficulty of explain- 

 ing natural immunity. 



A third theory is that the tissues are endowed 

 with some power of vital resistance to the develop- 

 ment of micro-organisms, similar to the vital 

 resistance to the coagulation of the blood, which is 

 supposed to exist in the lining membrane of the 

 healthy blood-vessel ; that in some species and indi- 

 viduals this exists to a high degree, and hence 

 their natural immunity But this does not explain 

 how one attack confers immunity from a subsequent 

 one. One would expect that the vital resistance 

 would invariably be lowered by a previous attack, 

 and increased liability be the constant result. 



