HEALTH AND DISEASE 47 



For a fuller account of this, aspect of the subject, for details 

 of the processes of inflammation, repair, and recovery, refer- 

 ence must be made to the larger textbooks on Pathology. 



Immunity, then, is a relative question. When an animal is 

 not susceptible to a given disease, it is said to possess a 

 natural immunity against that disease. On the other hand, 

 immunity may be acquired, sometimes by a process of gradual 

 training analogous to the gradual immunity acquired for 

 certain drugs such as opium, arsenic, alcohol, or tobacco ; in 

 other cases by having a single attack of the disease. 



Experimental or artificial acquireiL immunity is a very 

 important and interesting branch of the subject. In the 

 animal body we may produce what is called active immunity 

 by introducing the causal bacteria themselves, alive, weakened, 

 or dead, or their products. The deliberate inoculation of living 

 virulent bacteria, even in minute doses, is of course always a 

 risky proceeding, and is now practically never carried out in 

 the case of man himself, though this was one of the earliest 

 methods to be employed, as, for instance, when the virus from 

 the smallpox pustule was inoculated directly into the healthy 

 person in the hope of producing a mild protective attack of 

 the disease a method introduced from the East into this 

 country by Lady Mary Wortley Montague, but soon discarded 

 and prohibited by law owing to its dangers. 



The first great advance in such prophylactic or preventive 

 inoculation was Jenner's discovery of Vaccination, cow-pox 

 being caused by an organism closely allied to, or very pro- 

 bably identical with, that of smallpox, but so much weakened 

 by its passage through the cow as to be practically harmless 

 to man, yet producing a sufficient reaction in his tissues to 

 establish a very marked immunity against the terrible scourge 

 of smallpox itself. 



The term " Vaccination " has now been extended to include 

 the artificial inoculation of the weakened or dead organisms of 

 any of the infective diseases, and such methods are extensively 

 used as a protective measure for those who are liable to be 

 exposed to infection by typhoid fever, plague, and several 

 other diseases. Similarly, Vaccination may also be used in 

 many instances as a therapeutic measure, i.e. in the treatment 

 of a disease already present. Extraordinarily good results 

 have been thus attained in the treatment of boils and pustules 

 and other infections due to Staphylococci, especially in some 

 of the chronic cases which resist the ordinary modes of medical 

 and surgical treatment. Many other diseases are also now so 

 treated, especially when the organism is more or less localised, 



