IV. 



From a practical point of view, immunity against a tropical disease 

 can only be spoken of as sufficient when it protects against the majority 

 of various strains of the organisms forming the cause of the disease found 

 in such region, for which it is intended to make use of for inoculation 

 purposes. 



V. 



The immunity test for identifying two morphologically similar 

 organisms can only allow of a definite conclusion when the test is positive, 

 but not so when it is negative. 



VI. 



The seriim of animals which have recovered from a disease and have 

 been hyperimmunised to a certain extent acquires preventive properties. 

 East Coast fever is an exception, but the injection of blood from 

 an animal suffering from East Coast fever into susceptible ones does 

 not communicate the disease, and this may explain the fact. The 

 preventive action of a serum is principally pronounced against a 

 homologous strain, either of the same animal (virus derived from the 

 same immune animal which supplied the serum ; in some trypanosomiasis 

 and in canine piroplasmosis) or of difEerent animals and of different strain 

 (horse-sickness) ; it may be deficient against heterologous strains (strains 

 of different origin ; some trypanosomiases, surra, and horse-sickness) or 

 even against a homologous strain derived from the animal which supplied 

 the serum (some trypanosomiases). 



VII. 



Concerning the immunity itself, there are no essential differences 

 between that caused by bacteria and that caused by protozoa. The fact 

 that an apparently recovered animal acts as a reservoir has its analogy 

 in pleuro-pneumonia of cattle, although the two cases are not quite 

 identical. In the latter, riiorbid lesions are still present, in which the 

 infective cause is retained ; in the former, the cause remains in an 

 apparently healthy animal. 



