IMMUNITY. 171 



illustration is necessary. But not every infectious disease 

 leaves immunity behind; many, on the contrary, are fol- 

 lowed by no increased resistance, or if there is increased 

 resistance, it is very transitorj'; 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 re- 

 covery 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 

 injection 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 (p. 10). The nature of the 

 protection furnished by this virus has been the subject of 

 much controversy. The opinion of the present day incKnes 

 to regarding vaccinia as small-pox which has been modified 

 by passage through a relatively insusceptible animal. Cer- 

 tainly there are many analogies between the protection against 

 small-pox afforded by vaccination and the other examples of 

 artificial immunity mentioned below. 



This question cannot be settled with certainty until the 



