GENERAL BACTERIOLOGY 
59 
Such immunity—not congenital but brought about 
by having had one attack of an infectious disease—is 
called “naturally acquired immunity.” 
If it is acquired by artificial means, it is spoken of 
as “artificial acquired immunity,” and may be either 
“active or passive.” 
Active immunity is that variety of the artificially ac¬ 
quired immunity in which the patient receives an in¬ 
jection of dead bacteria (as in typhoid vaccination) or 
of the matter from a sore (as in smallpox vaccina¬ 
tion) ; in such cases the patient is really made to under¬ 
go a very mild attack of the disease against which he 
is vaccinated, with the purpose of producing protective 
substances which will render him insusceptible to the 
disease proper; this immunity is called “active” be¬ 
cause the patient takes an active part in the produc¬ 
tion of immunity—he himself elaborates the protective 
substances. 
Passive immunity is that variety of the artificially ac¬ 
quired immunity in which the patient receives, in the 
injection, not the dead bacteria or their poisons, but 
the protective substances themselves; for example, in 
the case of diphtheria, a horse is injected with the 
diphtheria exotoxin (in gradually increasing doses) 
until he has a vast number of protective substances, 
then his blood (in reality the blood serum) containing 
these protective substances is injected into a patient 
suffering from diphtheria; since the patient did not 
take an active part in the elaboration of the protective 
substances, this variety of immunity is called “ pas¬ 
sive” (on the other hand, the immunity produced in 
the horse by the injection of the diphtheria exotoxin is, 
of course, active, since the animal had elaborated its 
own protective substances). 
