144 BACTERIOLOGY FOR NURSES 



vidual natural immunity is, however, a more or less relative term ; 

 in fact, in the same individual slight factors such as exposure to 

 cold or fatigue may be sufficient to change the balance and convert 

 a condition of resistance into one of susceptibility. In some cases 

 resistance is so feeble that the equilibrium between health and 

 disease is easily disturbed ; in the case of tuberculosis in man the 

 body possesses sufficient natural immunity to resist small amounts 

 of infection, but this resistance is quickly broken down by any 

 influence which undermines the general vitality. Hard work, 

 mental and physical, which involves late hours and inadequate 

 periods of rest and recreation ; insufficient food and bad air all 

 tend to lower immunity and increase susceptibility to infection. 

 Exposure to wet and extreme cold is well known as a factor in the 

 etiology of colds and pneumonia. Experiments with animals 

 give abundant proof of these facts. For instance, chickens ordi- 

 narily immune to anthrax may become susceptible if their feet are 

 kept in cold water ; white rats, also usually immune to anthrax, 

 become susceptible after being compelled to turn a revolving wheel 

 until exhausted before they are inoculated. 



In epidemics of certain diseases many individuals escape, while 

 in other persons infection appears in varying degree of severity; 

 It is probable that the number of invading organisms, or their viru- 

 lence, or the channel of infection may account for these apparent 

 differences as well as varying degrees of immunity. 



Acquired Immunity. As its name implies, acquired immunity is 

 the converse of natural immunity ; it is acquired and not inherent. 

 Acquired immunity occurs in two distinct forms, active and passive. 



Active Acquired Immunity is resistance to infection due to the 

 activity of the body cells as a result of an attack of the disease in 

 question, or as a result of artificial inoculation with the specific 

 organism in a modified form, or its products. Immunity of 

 this kind is active in the sense that it occurs as the result of the 

 active struggle of the body cells against the invading parasites, 

 a struggle in which the foe is overcome and the body cells become 

 more resistant than they were before. Active immunity may 

 be gained by 



