IMMUNITY AND PROTECTIVE INOCULATION 511 
The anti-hog-cholera serum (while it is called serum it usually 
consists of the defibrinated blood of the immunized animal) is very 
extensively employed to protect swine against hog cholera. If 
properly used it is very efficient. The immunity established by the 
use of the serum alone lasts but a few weeks. 
The value of diphtheria antitoxin as an immunizing agent against 
diphtheria is well known. 
The simultaneous method. This consists in using a strong virus 
together with an immunizing serum. It is employed quite extensively 
against rinderpest, anthrax, and hog cholera. 
In case of rinderpest the animals are injected with a protective 
serum simultaneously with the virulent blood. The immune serum 
is obtained from animals that have recovered spontaneously from 
rinderpest or from cattle that have been immunized to it. The serum 
alone of animals that have recovered spontaneously possesses very 
slight protective properties unless very large doses are given. Kolle 
and Turner showed that if animals just recovering from an attack 
were injected with large quantities of the blood coming from animals 
suffering with a fatal attack, the protective power of their serum 
was markedly increased. This serum may be kept for a long time 
by adding a small quantity of carbolic acid. 
In rabies the method is reported to be most successful. Its essen- 
tial advantage over the other process is that it can be used with good 
results much later in the period of incubation. It has the additional 
practical feature that the number of injections is minimized. 
In anthrax it is reported to be giving much better results than the 
double inoculation with a weak and stronger vaccine as followed in 
the Pasteur method. 
The simultaneous method is used extensively in the protection of 
Swine against hog cholera. 
The use of bacterins. Heated (killed) cultures of bacteria have been 
used both for protection and therapeutic purposes. They have given 
as a rule better results when employed as remedies after symptoms 
have appeared than as purely prophylactic measures. 
Difficulties and dangers to be considered in vaccination. The 
results of the efforts that have been put forth to obtain control over 
the infectious diseases of animals show that with the victories there 
have been some failures. It is not always possible to know the exact 
degree of virulence possessed by the attenuated virus or vaccine, and 
again the resisting forces of animals vary even in different individuals 
