_House dogs_ as a rule suffer more severely, as their systems are more sensitive to the cold, and the resisting power to invading microbes is lessened. Country dogs and those living in the open air are hardier and more resistant and often have the disease in a mild form. _High bred dogs_ suffer more severely probably largely from the greater protection and care lavished on them and their consequent diminished power of resistance. Newfoundlands, great Danes, pointers, pugs, poodles, spaniels and greyhounds may be named as especially liable. The exposure to infection at _shows_, and in traveling by _rail_, _steamboat_ or _other infected conveyance_ must be considered as among the prominent causes. An exclusively _bread diet_ has been recognized as a predisposing cause, by reason of its lowering the stamina of the carnivorous animal. Any condition which induces _debility_ whether severe or continued disease, parasitisms, impure air, under feeding, improper food, rachitis, or scurvy, etc., must be admitted to operate in the same way. Any _catarrhal disease_ of the nose or bronchia is especially conducive to the affection by weakening the mucosa and making an easy entrance channel for the germs. _Change of climate_ is a strongly predisposing condition, which not only hastens an attack, but even, at times, arouses anew the susceptibility in dogs that have passed through a first attack. Dogs that suffered in England have had a second attack in India, and some have even had a third attack when brought back to Europe. Four attacks within a year, in the same dog, and without change of climate, are recorded by Friedberger and Fröhner. _Youth_ is much more susceptible than age, even apart from the immunity which comes to the mature dog from a first attack. Yet some puppies are insusceptible from birth. _Microbiology._ No one pathogenic organism has been proved to be the constant infecting agent, yet two classes of investigators have contended in favor of micrococci and bacilli respectively. _Micrococci_, 1 to 3μ in diameter, singly or in chains of 6 or 8 have been found in the blood, lungs, liver, kidneys and spleen (Semmer, Friedberger, Krajewski, Rabe, Mathis, Marcone, Meloni, Kitt). _Bacilli_ were found associated with micrococci by Semmer, Legrain and Jacquot, Laosson, Millais, Schantyr, and Galli-Valerio. As observed by the latter they were 1.25, to 2.5μ long, by 0.31μ broad and took the Gram stain. Cultivated on gelatine the colonies were waxy, lustrous points, which indented the gelatine without liquefying it. Cultures of the mixed bacteria (cocci and bacilli) and their inoculation on young dogs produced the symptoms of distemper with subsequent immunity (Laosson, Millais). Legrain and Jacquot claimed immunity, as resulting from inoculations with the cocci in pure cultures. Galli-Valerio, using the mixed cultures produced all the symptoms (pulmonary and cerebro-spinal) of distemper.