It is certain that on the introduction into the body of an inappreciable portion of the matter of cow-pox, or of small-pox, those specific forms of fever are produced. From these and similar facts it is inferred, that when putrescent or decomposing organic matter is introduced into the blood it acts as a poison and produces the phenomena of fever, and that all the predisposing causes of epidemics act in this way—by overcharging the blood with the products of decomposing organic matter. Strictly speaking, however, all that we really know is this—that where certain conditions exist, epidemics break out and spread; that where those conditions do not exist, epidemics do not break out and spread; and that where those conditions did exist, but have been removed, thereupon epidemics cease. We call those conditions Causes, Predisposing or Localizing Causes, but how they act, whether by accumulating decomposing organic matter in the blood, or in what other way, we have no certain knowledge. One further fact however is ascertained, that where any one of these predisposing causes is present, epidemics break out and spread just as readily as when all are present together. Where there is overcrowding alone, for example, epidemics break out and spread. Where there is decomposing filth alone, epidemics break out and spread; and so of the whole number. The removal of one of these causes, therefore, or the removal of two or three of them, will not suffice for safety; every one must be removed before there can be safety. This we know; all beyond this is conjecture, but as to the most probable of these conjectures, some who have thought on this subject believe that the preponderance of evidence justifies the conclusion that the predisposing causes may themselves become efficient causes; that instances in which they actually do so, are constantly passing before our eyes; that it is practicable to manufacture fever and even epidemic fever to any amount by placing a population under certain known conditions; that it is practicable to prevent the outbreak of epidemics altogether by placing the population under certain other conditions;[10] that the prevalence of the predisposing causes in particular localities, in certain intensities, is sufficient to produce local epidemic outbreaks; that the prevalence of such causes in such intensities, joined to some general conditions of the atmosphere, such as the meteorological conditions which have been enumerated, particularly those which favour the accumulation and concentration of the products of organic decomposition, are all that is required to engender wide-spread epidemics. Those who adopt this view contend that the existence of a primary cause as a distinct and separate entity is not necessary to account for the phenomena.