24 INFECTION AND IMMUNITY 



communicated by inoculation to man, to cattle, to 

 sheep, to guinea-pigs, rabbits, and mice, but the rat, 

 the dog, carnivorous animals generally, and birds are 

 immune ; glanders, which is essentially a disease of 

 horses, may be communicated to man, to the guinea- 

 pig, and to field-mice, while house-mice, rabbits, cattle, 

 and swine are to a great extent immune ; smallpox 

 is essentially a disease of man, but a modified form 

 of the disease may prevail among cattle (cowpox). 

 Susceptibility to infection also depends to a con- 

 siderable extent upon conditions relating to the indi- 

 vidual. It is well known that an attack of certain 

 infectious diseases protects the individual from subse- 

 quent attacks. This subject will receive attention in 

 the chapter devoted to " Acquired Immunity." But 

 in the absence of any such acquired immunity the 

 susceptibility of individuals of the same species dif- 

 fers considerably, and the same individual may be 

 more susceptible to infection at one time than at 

 another. Certain families or races are especially sus- 

 ceptible to infection by certain disease germs. Thus 

 the negro race is less susceptible to yellow fever and 

 to the malarial fevers than the white race ; on the 

 other hand, smallpox is exceptionally fatal among 

 negroes and dark-skinned races. In general it may 

 be said that when an infectious disease is first intro- 

 duced among primitive races, who, by reason of their 



