The reverse of immunity insusceptibility, and in speaking generally 

 about certain diseases having no influence on particular animals, we most 

 frequently use the term non-susceptibility ; thus indicating that the word 

 immunity has a more specific sense. It is more especially applied in such 

 instances where animals or certain classes of animals which are usually 

 susceptible to a certain disease have, by the process of recovering from 

 that disease, acquired such new quaUties as to render them insusceptible 

 against the further attacks of the same disease. 



For the purpose of this article, the term will be used in its restricted 

 sense ; for instance, a horse which has recovered from horse-sickness or 

 from piroplasmosis will be called immune against these diseases. For 

 the present the question will be left open as to the nature of such immunity ; 

 this will follow as a corollary from the deliberations. 



Complete Immunity. 



An immunity is complete when the cause of the disease has no influence 

 on the recovered animal under any conditions whatsoever ; that is to say 

 whether the test is appUed naturally in the form of an exposure, or 

 artificially by introducing the specific germs, and irrespective of the quan- 

 tity, manner of injection, and lapse of time succeeding the recovery. We 

 know of one disease where this has been observed to be the case, viz., in 

 rinderpest. In South Africa two epidemics of this zoonose have been 

 observed — the first during the years 1896-97, and the other during 

 1901-03. Animals which recovered in the first outbreak were refractory 

 to the second one, and it is admitted that in rinderpest immunity lasts 

 for life. 



After the fact was estabhshed that the serum of immune animals 

 had protective properties, use was made of this fact and immune oxen were 

 hyperimmunised to a great extent by injecting several litres of virulent 

 blood. In no instance under my observation did such an injection lead 

 to a breakdown of immunity. We may thus safely accept that immunity 

 in rinderpest acquired by recovery cannot be broken, neither naturally 

 nor artificially. 



Temporary Immunity. 



The immunity of pleuro-pneumonia in cattle has been made use of 

 in South Africa probably ever since that disease has been introduced. 

 It has been noted that an ox which has recovered from the disease is 

 immune. The expression pleuro-pneumonia indicates that the pathological 

 lesions are found on the lungs and the pleura. Although the primarv 

 seat is in the lungs, yet the disease is by no means specific for this organ. 

 It is rather specific for the lymphatic system, starting in the interstitial 

 tissue of the lung and involving the lung parenchym proper secondarily. 

 Therefore typical lesions can after artificial introduction of virus develop 



