58 
PRINCIPLES OF BACTERIOLOGY 
different races show different degrees of resistance to 
the same infection; our own domestic sheep are much 
less resistant to anthrax than the Algerian sheep; 
among the human beings we know that while the Amer¬ 
ican Indians and negroes are much more susceptible 
to tuberculosis than the white men, the latter are much 
more susceptible to yellow fever than are the negroes. In 
connection with this racial immunity the fact must not 
be lost sight of that we are also dealing with the dif¬ 
ferent hygiene conditions and customs which doubtless 
play a part in this question. 
Finally within the same race there exists great varia¬ 
tion in susceptibility and resistance among the individ¬ 
ual members, as seen, for example, in schools, when 
during an outbreak of an epidemic only a few pupils 
contract the infection. 
2. Acquired immunity is one which does not exist in 
individuals at birth, but which develops in them either 
as a result of having had an attack of an infectious 
disease or is brought about by artificial means. 
It is well known that many infectious diseases occur 
but once in the same individual, one attack usually con¬ 
ferring a lasting immunity against subsequent attacks; 
the following table from Zinsser’s Infection and Resist¬ 
ance (published by the MacMillan Co., 1914, p. 60) may 
be referred to in this connection. 
Infectious diseases in which one at¬ 
tack usually confers lasting im¬ 
munity : 
Plague 
Typhoid fever 
(Second attacks rare—in 2.4 
per cent cases) 
Cholera 
Smallpox {Second attack 
Chickenpox > 
Scarlet fever J vei T rare 
Measles—Secondary attacks 
quite rare 
Yellow fever 
Typhus fever 
Syphilis—Reinfection rare 
Mumps—Secondary attack rare 
Infectious diseases in which one at¬ 
tack does not confer lasting im¬ 
munity: 
Pyogenic infections. 
Gonorrhea 
Pneumonia 
Influenza 
Dengue fever 
Diphtheria ■ 
Recurrent fever 
Tetanus 
Erysipelas 
Beriberi 
Malaria 
Tuberculosis 
