CLASSIFICATION OF IMMUNITY 117 
Passive immunity is induced in the host by the injection of anti- 
bodies which have been devel()])ed in another animal. The recipient 
of these antibodies is protected only so long as they remain in the 
body. The immunity, however, is effective almost immediately 
after injection; there is no latent period. 
(6) Chemotherapy.— T\\e use of chemicals for preventing or modi- 
fying infection. For example, the use of quinine for malaria or the 
use of arsphenamine in syphilis. 
3. Mixed, Active and Passive Immunity.— Mixed artificially acquired 
immunity is induced by the simultaneous injection of specific anti- 
bodies and the weakened or attenuated virus; resistance to infection 
is usually increased at once (passive immunity), while at the same 
time the host begins to react to the virus and to produce antibodies 
thereto (artificially acquired immunity). Thus, diphtheria toxin- 
antitoxin appears to confer some degree of tolerance to infection with 
the diphtheria bacillus, and also leads to specific antitoxin formation 
within the body. It should be remembered also that the person may 
be sensitized to the horse serum by this process of immunization. A 
subsequent reinjection of serum for preventive or curative purposes 
may cause serum sickness: theoretically the possibility of acute ana- 
phylactic shock must be also considered in such cases. 
The factors which predispose the host to, or protect him from, inva- 
sion by microorganisms are usually varied and complex. Relatively 
simple explanations of the mechanism involved suffice to account for 
the phenomenon in specific instances, however. For example, frogs 
and hens are not naturally susceptible to infection with the anthrax 
bacillus, whose optimum temperature of growth is 37° C, yet infection 
can take place if the body temperature of either animal is brought 
to this level, as Pasteur showed nearly five decades ago. A change 
in environment may predispose to infection; the carnivora in their 
native state are quite resistant to infection with the tubercle bacillus, 
whereas in captivity they may succumb readily. Similarly, man 
placed in bad hygienic surroundings appears to be distinctly more vul- 
nerable to many infectious diseases than he is when his environment 
is more sanitary. Unhygienic conditions, however, are relatively 
complex in their reactions on man, for the attendant evils of over- 
crowding, underfeeding and increased exposure to infection undoubt- 
edly play a part. 
Heredity also appears to be an important factor in determining the 
average severity of infection in certain types of endemic disease. 
Measles is a common and usually fairly mild disease of childhood 
among civilized people. Among aboriginal populations, as those of 
the South Sea Islands where the inhabitants had not been exposed 
to measles previous to the advent of Europeans, the introduction 
of the virus resulted in a veritable plague during which large numbers 
of the people died. This phenomenon of hereditary acquired tolerance 
for specific endemic disease may conceivably be even more specific; 
