IMMUNITY. HI 



times a fatal dose of a very virulent micro-organism 

 will be neutralized by several times the amount of 

 serum which a single fatal dose requires, since in the 

 case of very virulent living bacteria whose virulence 

 is due to their ability to increase, it is not the organ- 

 isms which are introduced that kill but the millions 

 that develop from them. As a rule, the serum has to 

 be given before the bacteria introduced into the body 

 have multiplied greatly. After that period has elapsed 

 the serum usually fails to act, but some serums will 

 prevent further development even then. The immu- 

 nity conferred on a person from serum lasts from a 

 few days to several months, according to the amount 

 of serum injected. As in animals, it is strongest 

 immediately after absorption. An injection of bac- 

 terial poisons or the contraction of actual disease usu- 

 ally confers immunity from one to three weeks after 

 the infection, and lasts, according to the nature of the 

 infection, from one month to a year or more. The 

 serum loses all appreciable protective value as measured 

 in test animals in the usual doses before the person is 

 liable to infection. Repeated injections of serum con- 

 tinue this condition of immunity indefinitely. 



The use of serums having specific protective proper- 

 ties has been tried both in animals and man as a pre- 

 ventive of infection. In susceptible animals injections 

 of some of the very virulent bacteria, as pneumococci, 

 streptococci, typhoid bacilli, and cholera spirilla, can 

 be robbed of all danger if small doses of their re- 

 spective serums are given before the bacteria have 

 increased to any great extent in the body. If given 

 later they are ineffective. For some bacteria, such 

 as tubercle bacilli, no serum has been obtained of suffi- 



