104 ESSAYS ON BACTERIOLOGY. 



Modem bacteriological experiment has stepped in 

 at this point to enlarge our knowledge, revealing the 

 fact that some diseases are followed by a period of 

 immunity, distinct,, but so short-lived as to have es- 

 caped observation or demonstration. This seems to 

 be true of cholera. Erom such short periods we rise, 

 step by step, to reach diseases like measles, scarlatina, 

 typhoid and variola, the protection of which usually 

 extends over such a time as spans the ordinary dura- 

 tion of human life. Fortunate it is that immunity 

 is so often long-continued. Yet the fact remains that, 

 as a general rule, with passing years its energy declines. 



When we come to apply this rule to vaccination, we 

 shall see how the recognition but misunderstanding of 

 it at one time threatened to wreck that life-saving 

 agency. 



Third. Quantity and quality of the virus, or vigor 

 of the attack, is an important factor in the immunity 

 which shall follow. 



It is a common opinion, both professional and pop- 

 ular, that those who have experienced mild attacks of 

 a disease are more liable to recurrence than those in 

 whom the attack has been pronounced. Laboratory 

 experiment has again stepped in here with a contribu- 

 tion of more exact knowledge. It has been abun- 

 dantly shown in the laboratory, where the conditions 

 can be so thoroughly controlled, that the quantity 

 and activity of the virus or other protective agent 

 stand usually in direct ratio to the immunity which 



