200 MANUAL OF BACTERIOLOGY. 



Furthermore, as just stated, immunity probably also results 

 from intimate contact with diseased individuals. 



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 

 illustration is necessary. But not every infectious disease 

 leaves immunity behind; many, on the contrary, are followed 

 by no increased resistance, or if there is increased resistance, 

 it is very transitory; some even tend to increase susceptibility. 

 Examples of infectious diseases leaving no lasting immunity, 

 and even in some cases apparent increased susceptibility, are 

 erysipelas, diphtheria, influenza, pneumonia, gonorrhea. 

 In those cases in which immunity follows recovery from a 

 spontaneous attack of an infectious disease the process is 

 probably the same as that which takes place after intentional 

 inoculation. 



Immunity acquired after intentional inoculation is produced 

 either by inoculation with material which produces a mild 

 type of disease, as in vaccination for small-pox; or by giving 

 at short intervals inoculations of the disease-producing virus 

 of graded strength, beginning with greatly attenuated material, 

 and followed with stronger and stronger material, as in inocu- 

 lation for hydrophobia and anthrax; or by injections of larger 

 and larger amounts of bacterial poison, as in the production 

 of antitoxin for diphtheria and tetanus; or, finally, by the in- 

 jection of the antitoxin formed in the blood by the method 

 last mentioned, as in the treatment of diphtheria and tetanus, 

 for this, after all, is, in a way, a process of immunization. 



Small-pox and Vaccination. — The origin of vaccination 

 against small-pox with the virus of cow-pox has been de- 

 scribed in the historical sketch. The nature of the protection 

 furnished by this virus has been the subject of much contro- 

 versy. The opinion of the present day inclines to regarding 

 vaccinia as small-pox which has been modified by passage 



