1. The virus Tzaneen has in no instance been virulent for all mules injected; this virulency differs, so to say, from animal to animal; one particular animal seems to influence the virus in such a way that this virulency is either decreased or increased.
2. This reduction of virulency is by no means the result of the dose of virus or of method of injection; small and large doses, injected subcutaneously or intra-jugularly, equally fail to produce reactions.
3. This difference in virulency is either due to the virus itself or to the injected animal, but considering that animals which resisted quantities of virus of one particular type, subcutaneously or intra-jugularly, contract horse-sickness from a subsequent subcutaneous injection of a smaller quantity, it shows that it is not so much the resistance of the animal but the virulency of the virus of the given animal.
4. It is probably correct to conclude that both animal and virus must be in a certain relation to each other before a reaction can ensue.
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