176 MANUAL OF BACTERIOLOGY. 



CHAPTER VII. 



IMMUNITY. 



UNDER the title of immunity a number of nearly related 

 subjects may be discussed. It has already been shown that 

 the body is constantly liable to the attacks of pathogenic 

 microbes, which are endeavoring to effect an entrance. Some 

 of the defences of the body have been described. But it has 

 been found that there are other more subtle and at the same 

 time far more powerful weapons, which usually succeed in 

 repelling the invasion. This will answer the question often 

 asked, Why do we not all constantly have infectious dis- 

 eases ? In case bacteria do gain a foothold in the body and 

 produce what we call an infectious disease, we have to con- 

 sider the means by which the intruders are overcome. The 

 exemption from a second visitation which often follows one 

 attack of an infectious disease also needs to be accounted 

 for. 



Certain facts concerning immunity, some of which were 

 observed many years ago, are extremely interesting, but 

 very difficult of explanation. Even in the light of recent 

 bacteriological researches their interpretation is by no means 

 clear. The immunity which an individual who has suffered 

 from an attack of measles or scarlet fever possesses from a 

 second attack of the same disease is well known ; so also is 

 the immunity from small-pox which is conferred by vacci- 

 nation. Such an immunity is called " acquired." There is 

 also a " natural " immunity. Field-mice are susceptible to 

 glanders and house-mice are not. House-mice are suscep- 

 tible to mouse-septicemia and field-mice are not. Although 



