22 METHODS OF VACCINATION 



agent, since the blood frequently contains other infective materials 

 which would complicate the issue. 



In pleuro-pneumonia of cattle the severity of the disease is 

 lowered by altering the route of infection. In the natural disease 

 the infection probably enters by the lung, and its course is severe 

 and dangerous. Protection is conferred by inoculating virus from 

 the lung of an animal dead of the disease into the subcutaneous 

 tissue near the tail ; much local swelling results, and general 

 immunity is established. Perhaps, strictly speaking, this method 

 of induction of active immunity should be put in a class of its 

 own ; it is one in which a local is substituted for a general disease, 

 with the obvious result of greatly lessening its severity. 



The material used in the production of artificial immunity of 

 the type we are describing is sometimes called a vaccine. This 

 is undesirable, and it is advisable to use the word virus for material 

 containing the infective agent in its normal virulence, retaining 

 the word vaccine for that in which the bacterium has entirely lost 

 its power of producing the normal disease, whatever the dose and 

 whatever the channel of introduction. The term is a somewhat 

 unfortunate one etymologically, but it is in such general use that 

 it is hopeless to attempt to displace it. 



2. By the use of living cultures of pathogenic bacteria of 

 diminished or altered virulence i.e., of a living vaccine. There 

 are as many modifications of this method as there are ways of 

 mitigating the virulence of a culture, and different methods are 

 applicable to different diseases. 



(a) By the use of vaccines diminished in virulence by passage 

 through animals. The most important example of this is, of 

 course, Jennerian vaccination. It would take us too far to 

 examine the evidence in favour of this view, but it may be taken 

 as fairly proved that ordinary lymph vaccine consists of a culture 

 of the smallpox organism modified by passage through " calves, 

 the modification being of such a nature that it has lost its power 

 of producing a general disease (smallpox), but retained that of 

 causing a local one (vaccinia) otherwise similar in nature. 



We have already referred to the decrease in the virulence of the 

 bacillus of swine erysipelas on passage through rabbits, and the 

 use of these mitigated cultures as a vaccine for pigs. This is a 

 better example of the type of immunity we are considering, since 

 it has to do with a known organism. 



(b) By the use of vaccines in which the virulence is diminished 



