_Accessory Causes._ The recognition of the one essential cause in the microbian invasion, need not exclude as accessory factors the many unwholesome conditions which have long been recognized as contributing to the severity of epizoötics. As the seed requires the rich field, the rain and sunshine to bring it to an abundant harvest so the microbe of equine influenza flourishes best where the conditions are most favorable and the antagonisms least. The _chill_ which comes from a sudden extreme fall of temperature, or the standing in a cold draught when wet or perspiring, lays the system open to this as to other microbian invasions. The _electric tension_ preceding a thunderstorm, to which many of the lower animals are excessively susceptible equally prepares the system to succumb to the germs. It may here be noted that September 1872, the last days of which witnessed the start of the great epizoötic, had no less than eleven thunderstorms, while in September of the previous year there were but two in the vicinity of Toronto. It is just possible that the great and frequent electric tension, lowered the animal vitality, allowing a violent invasion by the hitherto slumbering germ, and gave to the latter that encreased potency which sent it forth on that year of almost unparalleled epizoötic record. The _high barometer_ and _low dew point_ similarly affect the animal economy and encrease receptivity to disease. Rain fell at Toronto 16 days in September 1872 and but 8 days in September 1871. _Impurities in the air_ whether originating in volcanic eruptions, telluric emanations, close, filthy overcrowded buildings or compartments or large collections of decomposing organic matter, impair the animal vigor and lay the system open to a more violent attack. For this among other reasons epizoötics of equine influenza are nearly always more deadly in the closely packed city stables than in the pure country air. _Overwork_ and _poor irregular feeding_ and _watering_ pave the way for debility, prostration and severe invasion. _Sudden vicissitudes of temperature_, which are so common in _spring_ and _autumn_, associated as they are with the _shedding and growth of the coat_, materially encrease susceptibility and sometimes determine an encreased severity in the attack. _Youth_ has its influence, even if it means only that the system that has never before been exposed to the poison, retains all its native susceptibility, and has none of that acquired immunity which comes from a previous exposure to the virus and successful resistance.