INTRODUCTORY AND GENERAL 21 



however, for the repeated injections of vaccines which are so mild 

 as not to cause any noticeable general and very little local reaction 

 may induce a high degree of immunity. 



The main methods in which active immunity is acquired are 

 these : 



i. A natural attack of the disease, or an attack which is natural 

 in course, but of artificial induction. The only example of the 

 latter in human medicine is the now disused practice of smallpox 

 inoculation, in which the person to be protected was inoculated 

 with the disease, which ran a perfectly usual course, and was not 

 infrequently fatal. As a rule, however, it was milder than the 

 naturally acquired smallpox, since the infective material was 

 taken from a favourable case, and the operation performed when 

 the patient was in good health and able to get proper attention 

 from the outset. Probably, too, the severity of the disease was 

 somewhat modified by the fact that the virus did not reach the 

 body by the usual route. But the infection was ordinary small- 

 pox, and might start an ordinary epidemic. 



The process is used to a much greater extent in veterinary 

 practice, where an occasional death due to the induced disease is 

 of comparatively little importance if thereby the outbreak can be 

 controlled or the great majority of the flock saved. As a rule, 

 an attempt is made to render the attack as mild as possible, 

 either by (a) limiting the amount of the infective material used, 

 or (b) by introducing it in an abnormal way, or (c) inoculating 

 animals at a time when they are found to be least susceptible, or 

 by a combination of these methods. Thus Texas fever is a 

 disease of cattle due to a protozoon (Piroplasma bigeminum) which 

 is conveyed by the bites of ticks. One of the methods used for 

 the protection of cattle in infected districts is to expose calves 

 whilst still milk-fed to the bites of a few infected ticks ; another 

 is to inject blood from diseased animals (containing the parasite) 

 in small doses direct into the jugular vein. In favourable cases 

 the result is a severe attack of the disease, which, however, is 

 rarely fatal, and is followed after a time by complete immunity. 

 In some cases the disease is but slight, and in them a second or 

 even third dose, in each case larger than the preceding, is required. 

 The mortality from the injections is from 3 to 10 per cent., whilst 

 that of untreated animals in infected areas is about 90 per cent. 



A similar method is in use for combating rinderpest, but here 

 bile from an animal dead of the disease is used as the infecting 



