VI'RUS (the Let. for a poisonous liquid) is a term used in medicine to signify those mysterious poisonous agencies which produce zymotic diseases (q.v.), such as small-pox, measles, scarlatina, the various forms of continued fever, ague, whooping-cough, cholera, syphilis, glanders, hydrophobia, etc. While each of these morbid poisons (as they are frequently called) has a definite and specific action, they collectively obey certain laws. For example (1.), their actions are variously limited, some affecting only one organ or system of organs, while others involve two or more organs or system of organs. Thus, in bronchocele or goiter, we have an example of a poison acting only on the thyroid gland, while iu whooping-cough and hydrophobia all the organs supplied by the pneu mogastric nerves (q.v.) are affected, and, in paludal or malarial poison most of the organs. (2.) Morbid poisons, like medicines and ordinary poisons, have their period of latency, which, however, here is usually much prolonged. While a medicine, e.g., is seldom longer than a few hours in exhibiting its effects, the poisons of scarlatina, measles, and small-pox remain latent in the system for at least seven, ten, and sixteen days respectively; while that of paludal fever and hydrophobia may be dormant for a year or upward. (3.) When several tissues or organs are acted on, the actions may be simul taneous, but are more commonly consecutive, a considerable interval often elapsing between the attacks. (4.) Another law of morbid poisons is, that two may co-exist in the same person; for example, small-pox and cow-pox have often been seen at the same time in the same person. In this case, each disease runs its course unaffected by the other; but most commonly, when two co-exist, one lies latent while the other runs its course. Thus, a case of intermittent fever may suddenly subside, and small pox make its appearance; on recovery from this disease the intermittent fever may return.
Among the peculiarities presented by morbid poisons, the following points must be noticed: (1.) In experiments made on the inoculation of the small-pox virus by Dr.
Fordyce, it was found that extremely diluted poison, if it acted at all, produced the same effects when introduced into the system as the concentrated virus. Hence it may be inferred that the intensity of the disease is not proportional to the amount of virus received into the system. (2.) Women in child-bed may not only engender a special poison of this class—that of puerperal fever—but are highly susceptible of these poisons, and almost always succumb to their action. (3.) Another peculiar law of morbid poisons, and one wholly unknown in medicinal substances, is, as Dr. Aitken remarks, " the faculty which the human body possesses of generating to an enormous extent a poison of the same nature as that by which the disease was originally produced. A quantity of small-pox matter not so big as a pin's head will produce many thousand pustules, each containing fifty times as much pestilent matter as was originally inserted; and the miasmata secreted by one child laboring under whooping-cough are sufficient to infect a whole city." A remarkable illustration of the development of syphilitic poison from a single infant over a whole province is given in the article SYPHILIS. (4.) A still more remarkable fact is, that many of these morbid poisons possess the property of never occurring more than once in the life of the seine individual. This is the case with scar latina, measles, small-pox, whooping-cough, and (to a less extent.) typhoid and typhus fevers. (5.) This class of poisons is powerfully influenced by climate, and probably by the nature of the soil. Thus, the severe forms of typhus so common in this country are hardly known in warmer climates, and the influence of cold weather on cholera and plague are well known.—For further information on this subject, the reader is referred to Dr. Robert Williams On ,Morbid Poisons; ; and to the chapter on zymotic diseases, in the first volume of Aitkeu's Science and Practice of Medicine, from which we have drawn most of the preceding remarks.