VIRUS (Latin, "a poisonous fluid"), a term used in medicine to designate the materies morbi of zymotic diseases. It more specially designates those peculiar poisonous matters which can reproduce themselves under favoring conditions to an endless degree. The poison of the cobra is a specific virus which, when in troduced into the human system, acts as a most virulent poison; but the poison is not multiplied within the human sub ject, and one person affected by the poison cannot communicate the disease to another. In like manner, morbid prod ucts from decaying vegetables under certain conditions of heat and moisture may possibly originate the virus of mala rial fever; but the virus is not propa gated within the human organism, or, at all events, never in such a form as to render it capable of producing the same disease in others. By some the virus of the contagious or infectious diseases is supposed to be a contagium vivum sett animatum, the theory being that the virus consists of living beings or low or ganisms. Such views have been advo cated by Kircher, Lancisi, Vallisneri, Reaumur, Linn& 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 specificness. Experience and observa tion tend to confirm the hypothesis that each specific disease breeds true, though, in the course of 1,000 years, it is possible that the 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 spontaneously, but are propa gated each from its own kind, though some contend that they do not originate, even in our own day, spontaneously or autochthonously. Another remarkable peculiarity belonging to many, but not to all, diseases propagated by a specific virus, is that a single attack of the dis ease successfully surmounted produces absolute or relative immunity for a cer tain length of time, or even for the re mainder of life. See BACTERIA.