Conclusion:-The immunisation against a polyvalent virus with an adequate serum produces immunity which protects against any of its constituents. The immunity is strong enough to prevent a reaction in the majority of cases, and strong enough to pass the reacting animals through a mild disease. Compared with the results obtained by the injection of a mixture of sera and their adequate vira, the immunity of the polyvalent virus and serum protects better against the constituents of the said polyvalent virus. I must state here that the reactions noted after the injection may not always be due to the injection, but may be of a coincidental nature, or the reaction may not necessarily be a horse-sickness reaction. At the same time, we have no other means of controlling horse-sickness reactions except by the thermometer, inasmuch as slight reactions are not accompanied with characteristic symptoms. For practical purposes, however, I consider that the immunity obtained by a polyvalent virus will, in practice, protect better than that obtained by an individual strain of virus.
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