MODE OF SPREAD OF INFECTIVE DISEASES. 755 



organisms. Only those parasites which have exactly the 

 same seats of invasion can come into play in causing a 

 reactionary alteration of these seats. It further becomes 

 intelligible that complete immunity against later in- 

 vasions is at times obtained by locally limited affections 

 which have caused an almost unnoticeable disturbance 

 of the whole body, for it is only a certain local affection 

 of the seats of invasion which seems to be of importance 

 for the production of the immunity. It is to be hoped 

 that experimental investigations will soon afford a more 

 sure basis for the attempt to explain immunity ; as yet 

 the explanations are quite hypothetical, and do not meet 

 all cases, but only apply to that group of infective dis- 

 eases which have specific points of invasion. 



The experience as to the action of pestilences, and Protective in- 

 more especially the knowledge of the protective effect of ot 

 vaccinia against small-pox, which has been gained by ex- 

 periments on millions of men for almost 100 years, have 

 demonstrated that the acquired immunity is in many 

 respects the most important factor in preventing the 

 devastating action of the infective diseases. In vaccina- 

 tion we artificially transmit organisms which are remark- 

 ably like the infective agents of small-pox, but which 

 excite in man only a mild and local disease. By this In variola, 

 inoculation we can prevent the invasion of virulent in- 

 fective agents of the same kind ; and such a vaccination 

 is of great value, especially for variola, because, as we 

 know, one attack of this disease gives a very long con- 

 tinued and certain immunity ; because, further, the at- 

 tenuated vaccine material preserves its slight degree of 

 virulence with great constancy ; and because as a result 

 we need neither fear injury from the inoculation, nor 

 uncertainty in the result. 



During recent years only few, and at the same time 

 crude and uncertain attempts have been made to extend 

 protective inoculation to other infective diseases, espe- 

 cially in the case of domestic animals, but since the year 



