Young animals were more susceptible than adults and although the most susceptible age varied with the epidemic phase, it corresponded in general with the period of weaning. The influence of physiological status was further indicated by the increased susceptibility incident to lactation.

Racial variations in susceptibility were obscured by age factors in young animals, but were of profound importance in the adult population and formed the most significant feature of the analysis. A high degree of conformity was found in the susceptibility of racially related breeds, and this similarity in behavior increased with the proximity of relationship. Moreover, a study of the hybrids obtained from the crossing of pure breeds showed that two separable groups of hereditary factors were concerned in the determination of breed susceptibility, one group consisting of essential racial characters, the other of constitutional factors incorporated in the stock by chance association, and that the final expression of susceptibility or resistance was the result of their combined interaction.