lyo MANUAL OF BACTERIOLOGY. 



Resistance to Infection. — Immunity from infectious dis- 

 eases is either natural or acquired, active or passive. 



By natural immunity is meant the inherited power pos- 

 sessed by certain races or individuals, independently of size, 

 habits or surroundings, to resist infection to which other races 

 or individuals are subject. This is illustrated by many ex- 

 amples. Rats are ordinarily insusceptible to anthrax, whereas 

 mice, guinea-pigs, sheep, cattle — in short, most animals — are 

 very susceptible. Mice are not susceptible to diphtheria 

 poison on inoculation, while horses, sheep, goats, guinea-pigs 

 and many other animals are very susceptible. Even very 

 nearly related species or varieties often show difference in 

 susceptibility. House-mice are susceptible to mouse septi- 

 cemia, field-mice are not. With glanders the reverse is the 

 case in these species of animals. Negroes are insusceptible 

 to certain diseases to which white persons are very subject, 

 and also the reverse. 



Instances of individual immunity are seen in every epidemic, 

 where persons escape when they are under the same conditions 

 as those who have contracted the disease. Instances also 

 occur in which nurses and others thrown with cases of highly 

 infectious diseases escape. Some of these cases, it is true, 

 belong more properly to the category of immunity acquired 

 by recovery from an attack, since nurses and others thrown 

 in contact with an infectious case may suffer a very mild attack 

 or even, it is probable, become immunized without showing 

 any symptoms of disease. Nurses and physicians have been 

 found with diphtheria bacilli in their throats and yet not show- 

 ing any symptoms. Of course, these persons may have had 

 natural immunity, but it is equally possible that they may 

 have become gradually immunized. 



Acquired immunity follows recovery from a spontaneous 

 attack of certain diseases, and it also results from intentional 

 inoculation. Immunity after recovery is so familiar that no 



