IMMUNITY. 111 
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. Asa 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- 
