752 MODE OF SPREAD OF INFECTIVE DISEASES. 



power, and can be distributed by the most various modes 



of transport, establish themselves, however, chiefly in 



the lungs or in the intestine, in other words, in places 



which are protected by special arrangements against the 



entrance of infective agents in a manner varying in 



individual cases ; and the danger of contagion is still 



less when the energy of the infective agents is so slight 



as compared with that of the cells of the body that they 



can only reach the place of their development when the 



epithelium has been specially prepared and weakened. 



Influence of The influence of individual predisposition is most 



tionTn the 081 " strikingly seen in the case of tuberculosis ; in this 



spread of disease we learn from experience that the greater or 



tuberculosis. x 



less accumulation of resistant infective agents in human 

 surroundings plays a relatively subordinate part in the 

 spread of the disease ; on the other hand, the state of 

 the protective arrangements at the points of entrance, 

 and especially the state of nutrition of the mucous 

 membrane, has such a definite and important influence 

 that it has become the rule to pay the greatest attention 

 to the removal of the individual predisposing factors (by 

 good nourishment, increased respiratory movements, 

 &c.), and not to attempt the careful removal of the 

 infective agents, as is done in the case of other infective 

 diseases. 



5. Acquired Immunity and Protective Inoculation. 



Of special importance is the immunity acquired as 

 the result of recovery from the same disease, or from 

 one caused by similar or attenuated exciting agents. 

 Relapsing Such immunity is not acquired in the case of all 



infective diseases. Erysipelas, pyaemia, gonorrhoea, 

 relapsing fever, pneumonia, and malaria, frequently 

 show relapses even within a short time after recovery 

 from the first attack. Other diseases are followed by 

 immunity for some time, but never without exception, 

 nor to a similar degree, in the different species of 

 animals ; for example, anthrax may occur repeatedly in 



