506 IMMUNITY AND PROTECTIVE INOCULATION 
Methods for producing acquired immunity. Immunity may be 
produced in several ways other than by causing the individual to 
pass through an attack of the disease caused by natural infection. 
These methods may be summarized as follows: 
By inoculating the individual with a non-lethal dose of a strong 
virus. This is practiced in immunizing cattle against Texas fever, 
sheep pox and contagious pleuro-pneumonia. 
By inoculating the individual with an attenuated virus. This is 
practical in anthrax, blackleg, chicken cholera, rouget, rabies and 
bubonic plague in man. 
By inoculating the individual with a vaccine consisting of the virus 
of the disease modified by passage through another species of animal, 
as vaccine for small pox. 
By the injection of toxins. In practice this is used for immunizing 
animals, especially horses, against the virus of the diseases for the 
purpose of procuring antitoxin from their blood, as in the preparation 
of diphtheria and tetanus antitoxins. 
By the injection of antitoxins. These are used to immunize ani- 
mals against toxins, and children against natural infection, as in 
diphtheria and tetanus. 
By the injection of defibrinated blood or serum from immune or 
hyperimmune animals for prophylactic purposes. The use of anti- 
hog-cholera serum to immunize swine against hog cholera is an exam- 
ple of this use of serum. 
Active and passive immunity. Acquired immunity may be active 
or passive. If the organism of the disease participates in the process 
of bringing about increased cell activity the resulting immunity is 
active. Such immunity is acquired at the expense and often at the 
risk of the individual acquiring it. 
Passive immunity is produced by the injection of the serum or 
antitoxin of animals already immune. 
It involves no active generation of protective substances on the 
part of the animal. The passively immunized animal is simply the 
recipient of substances formed in the body of other animals and trans- 
ferred to it. 
Active immunity is slow in its development, is more or less danger- 
ous to produce and it always attended with at least some discomfort. 
It varies in the time it lasts but usually it is quite persistent, continu- 
ing from a few weeks or months to several years. Passive immunity 
is quite rapidly produced, is attended with little or no danger and 
