IMMUNITY 55 



were susceptible and others immune to certain dis- 

 eases, but the line is now less sharply drawn and 

 differences are attributed more to differences of 

 habits, pursuits, and opportunities for infection 

 than to racial characteristics. Individuals vary 

 greatly in their powers of resistance, and members of 

 the same family subject to the same environment 

 often exhibit marked differences in their susceptibil- 

 ity to disease. Epidemics of typhoid fever afford 

 excellent examples of the natural immunity of cer- 

 tain persons; for while all may use the same polluted 

 water, not all are infected. 



It is said that the small animals used in the bacte- 

 riological laboratories exhibit the same individual 

 differences to a less degree. 



Acquired immunity may be active or passive. 

 Active immunity is due to the direct participation 

 of the organism concerned, as immunity to smallpox 

 is acquired by an attack of the disease. 



Passive immunity is acquired by the transfer of 

 protective substances formed in the body of another 

 animal to the body of the person to be protected. 

 The protection afforded by the use of diphtheria 

 antitoxin is an example of immunization by bac- 

 terial products; the horse is actively immunized by 

 the repeated injection of increasing doses of diph- 



