_Close_, _foul air_, _bad ventilation_, _imperfect sewerage_, and _overcrowded stables_ not only contribute strongly to infection but tend to aggravate the cases. _Underfeeding_ and _overwork_ act in the same way and in this connection may be named the _excitement_ and _exhaustion_ attendant on a _long journey by rail_. This, like the foul, crowded stable, furnishes many more opportunities for infection and reinfection, so that the invasion of the exposed animal system is all but certain. Infection clings to the _loading banks_, _yards_, _feeding stables_, _mangers_, _troughs_, _buckets_, _cars_, _litter_, and _manure_, so that young horses shipped from the west to the Atlantic Coast States, very frequently come down with contagious pneumonia, and contaminate the stables in which they are placed. Peters suggests that the germ is preserved in the _soil water_, so that after apparent subsidence it may be again brought to the surface in time of rains or freshets, to start a new epizoötic. _Convalescent horses_ may carry the germ for weeks, on the mucosa or in sequestra in the lungs, and contaminate horses with which they come in contact. It is remarkable that the contagious pneumonia is far less diffusible on the air than influenza, so that it is much more constantly the result of direct contact of a sound, with an infected animal, or with a place or thing that the sick animal has contaminated. It therefore spreads much less rapidly, remains confined to individual stables for a length of time, and in the absence of active interchange of horses tends to die out of its own accord. As the infection is not generally and speedily acquired, so immunity fails to become general, and the infection tends to fix itself permanently in places where many strange horses congregate, (market stables, sale stables, livery stables, etc.), and the constant influx of fresh animals keeps the flame burning by accessions of fresh fuel. In such cases it is manifest that the germ outside the animal body either rests in a dry condition, or lives as a saprophyte in earth or organic matter, and often loses much of its virulence. Under such circumstances animals that would prove readily susceptible to a virulent germ, prove nonreceptive to this resting germ, until under some special devitalizing influence, like exposure, exhaustion or local disease, it finds its opportunity and the weakened system succumbs. Then, acquiring new force through its life in the debilitated system, it starts on a recrudescence, and an epizoötic is mistakenly supposed to have started without a preëxisting microbian cause.