_Historic Notes._ This affection was formerly confounded with equine influenza, and it was only in the second third of the nineteenth century that the differentiation was attempted. S. Prangé describes this as a special epizoötic disease in the French hussars in 1841, Leconturier in Belgium in 1845, Seidamgrotzky in Germany in 1882 (strongly emphasizing the contagion), and Dieckerhoff in Berlin in 1883. The latter showed that horses, recovered and immune from influenza, still contracted brustseuche and perished. This distinction was fully corroborated by Lustig, Cagnat (1884), Brun, Delamotte (1886), Jolly, Benjamin, Leclainche, Trasbot, and others. The presence of a particulate, living, self-multiplying cause (microbe) was recognized as the essential condition of the disease (as we still recognize the necessity for such an organism to explain rabies) though the micro-organism itself was as yet undiscovered. At the same time many concurrent factors had to be considered as accessory in different cases. _Accessory Causes._ _Young horses_ often show a greater susceptibility than older animals, mainly because they retain all the unimpaired susceptibility of the colt, while old horses have already passed through the disease and become immune. On the other hand, in the absence of acquired immunity, the older, _worn out and debilitated animals_ are the most susceptible and tend to have the disease in its worst form. _Susceptibility_ and _immunity_ are therefore more important factors than mere age. Immunity usually lasts for several years, or throughout life, yet in some animals, or under given conditions, it is overcome much earlier. _Inclemency of the weather_, or special exposure of any kind, as in severe rainstorms, or working with the feet and legs in water, may become the occasion of an attack. Exposure to cold _northwest storms_ (America, Atlantic Slope), or northeast (Europe), _standing without blanket_ in a temperature at zero, confinement in _draughts of cold air_ between doors or windows, without clothing and after severe exercise, weaken the whole system and increase susceptibility. The presence of a _nasal or bronchial catarrh_, or of another debilitating disease may act in the same way. The weakened tissues seem to invite the entrance of the germ. Palat, Boiteux and Trasbot found that horses with _local sores_ or _suppurations_ fell readier victims than others;—perhaps the germs entered by the traumas; perhaps the tone of the whole system was lowered, so that the resistance was lessened.