VIRUS, morbid poison; the contagion of an infectious disease. It is the morbific principle, associated with a germ that is the medium for communicating disease between men and animals. The term more especially designates those peculiar poisonous matters which can re produce themselves under favorable conditions to an endless degree. The poison of the cobra is a specific virus which, when introduced into the human system, acts as a most virulent poison; but the poison is not multiplied within the human subject, and one person affected by the poison cannot communicate the disease to another. In like manner, morbid products from decaying vegetables under certain conditions of heat and moisture may possibly originate the virus of malarial fever; but the virus is not propogated within the human organism, or, at all events, never in such a fortn as to render it capable of producing the same disease in others. By some the virus of the contagious or infectious disease is supposed to be a contagion,' eitium seu oninsatum, the theory being that the virus consists of living beings or low organisms. Such views have been advocated by Kircher, Lancisi, Vallisneri, Reaumur, Linne, Henle, Roberts and others; and although the theory of a contagium vivum is not as yet complete, the discussion of it is the most important which has ever engaged the attention of medical men. The most prominent characteristic of
each specific virus is that it can reproduce itself within the human organism, and to an unlimited extent, each virus preserving its own specific ness. Experience and observation tend to con firm the hypothesis drat each specific disease breeds true, though, in the course of ages, it is possible that changes within certain limits may take place, as is. the case in animals and plants. The natural conclusion follows that diseases of this class do not originate spontane ously, but are propagated each from its own kind, though some contend that they do not originate, even in our own day, spontaneously or autochthonously. Another remarlcable pe culiarity belonging to many, but not to all, diseases propagated by a specific virus, is that a single attacic of the disease successfully sur mounted produce.s absolute or relative im munity (q.v.) for a certain length of time, or even for the remainder of life. Others hold the theory that the germ is a result not a cause of the poison and disease. See Bacrdans;