IMMUNITY. — 107 
variable degree is deleterious to their further growth. 
It also gives rise to substances which neutralize the 
poisonous effects of the bacterial products. This im- 
munity may take place in various ways: ~ 
1. Through recovery from disease naturally con- 
tracted or from infection artificially produced. This 
immunity may be slight, as after recovery from 
erysipelas or pneumonia, marked for a short period of 
time, as in diphtheria and typhoid fever, or prolonged, 
as after scarlet fever or syphilis. 
2. By the injection of the bacteria into tissues not 
well suited to their development, as the injection of 
typhoid bacilli or cholera spirilla into the subcutaneous 
tissues. Here a mild local infection follows, with con- 
siderable resulting immunity. 
3. By the injection of micro-organisms attenuated 
by heat, chemicals, or other means. In this case a 
local or general infection of the animal is produced, 
of moderate severity, as a rule, and the immunity is not 
as marked and lasting as after recovery from a more 
serious attack; but it is, nevertheless, considerable. 
The inoculation of sheep with the attenuated anthrax 
bacillus and the use of vaccination in man are examples 
of this method. 
4. By the injection of the unaltered chemical con- 
stituents of the dead bodies of bacteria and of the 
chemical products which they elaborate and discharge 
into the surrounding culture media during life. Smith 
and Salmon proved that by repeated injections of the 
filtered bouillon cultures of the hog-cholera bacillus a 
considerable immunity may be produced against the in- 
vasion of this bacillus. Similar results have followed 
the injections of dead cultures of typhoid and anthrax 
