IMMUNITY AND PROTECTIVE INOCULATION 505 
most striking examples of this are cases of recovery from small pox 
and yellow fever in man and Texas fever in cattle. The exanthema- 
tous diseases leave the individual with more or less immunity. In 
certain other infectious diseases there is little increased power of 
resistance imparted to the individual that has recovered from the first 
attack. In such diseases as diphtheria, the duration of the immunity 
resulting from a natural attack is variable. 
The period of duration is variable in artificial immunity. 
The fact that individuals that had recovered from certain diseases 
were rendered immune to a second attack led Pasteur and others to 
inquire into methods for artificially immunizing animals against the 
infections most destructive to them. Pasteur found that inoculating 
animals with attenuated virus* would immunize them against the 
strong virus or naturally acquired infection. He succeeded with 
swine erysipelas, chicken cholera, anthrax and later with rabies. 
Arloing, Cornevin and Thomas introduced a successful method of 
preventive inoculation with attenuated virus against black leg. 
The next procedure was a line of investigations directed toward the 
production of immunity by the use of heated cultures of the bacteria 
(bacterins) and the toxins in filtered cultures. The first of these was 
an immunization of pigeons against B. suipestifer with the use of 
heated bouillon cultures by Salmon and Smith in 1886. This line of 
investigation led eventually to the immunizing of animals experimen- 
tally with the toxins of certain virulent pathogenic bacteria such as 
those of diphtheria and tetanus. 
Another method that has been extensively tried experimentally 
with the bacterial diseases, but usually without success, is the use of 
non-lethal doses of virulent virus. It was somewhat successful in 
contagious pleuro-pneumonia of cattle. With certain of the proto- 
zoan diseases, such as Texas fever, this method is more satisfactory. 
It has been found that the blood serum of animals that are immune 
to certain bacterial diseases possesses antitoxic properties by which 
it is able to impart immunity to healthy susceptible animals, or to act 
as a therapeutic agent for those already affected with the same 
disease. Diphtheria antitoxin is a striking illustration of this. 
*This principle was exemplified centuries before in the far East where inoculation 
with small pox virus (material from the pustules) was practiced whenever small pox 
occurred naturally in a very mild form. Lady Mary Wortly Montague is said to have 
introduced this practice into Europe about 1718. Later, 1796, Jenner, after thirty 
years of labor, introduced the practice of inoculating human subjects with the virus of 
cow pox. This is known to-day as vaccination and the vaccine is prepared at the 
present time from calves. In 1839, Thiele showed that the disease known as cow pox 
was small pox in cattle. 
