Zymosis (Gr. Vuooig Fermentation), a term used in speculative pathology to denote the action of a peculiar and little known process analogous to fermentation. From remote antiquity various hypotheses have prevailed to account for the morbid changes of the blood in epidemic, endemic, and infectious or contagious diseases; but most of them have been so inseparably connected with the old humoral pathology that they received little consideration till rendered plausible by the researches of modern chemists, who assume the analogy of yeast in producing fermentation and a virus or poison producing its effects upon the system through the blood. According to the views of some, a zymotic change of the blood is due to catalysis, or continuous molecular action: for example, a decomposing organic molecule is introduced into the human body, and by a law of catalysis, induction, or contact, this molecule or germ imparts its own motions to other molecules with which it may come in contact. Chemists have defined this change to be "decomposition by contact" or the "action of presence." The term zymosis was introduced by Dr. William Farr, and includes epidemic, endemic, and contagious diseases, which are enumerated further on.

An illustration of this law is the power which small quantities of certain substances possess of causing unlimited quantities to pass into the same state. Other analogies are the contagion of motion, as witnessed when a stone is dropped into water, the diffusion of heat from molecule to molecule, the phenomena of crystallization, or the solution of an alloy of platinum and silver in nitric acid, when the platinum, which under ordinary circumstances is insoluble in nitric acid, takes on the action that is transmitted through the atoms of silver. A more remote analogy is the setting in motion of one multiplying wheel by another, or the extension of a conflagration to surrounding combustibles. An illustration more to the point is the molecular motion that takes place in the modern operation of skin grafting. - We are still ignorant of the different viruses, contagions, poisons, miasmata, etc.; but it can be shown, in attempting to trace some of their phenomena, that the introduction of putrid or contagious matter into the animal system gives rise to factitious diseases having all the characteristics of essential fevers. The following observations have been adduced in reference to this point.

Subjects in anatomical theatres frequently pass into a state of decomposition, which is communicated to the blood of the living body. Putrefying blood, brain, eggs, etc, laid on recent wounds, cause vomiting, lassitude, and death after a longer or shorter interval. Numerous experiments have demonstrated that putrid matter injected into the blood of healthy animals gives rise to a set of symptoms analogous to typhus. Injecting yeast or sugar into the circulation excites many of the ordinary kinds of fermentation, giving rise to a disease like typhoid fever. A universal observation is that the origin of epidemics is often to be traced to the putrefaction of large quantities of animal and vegetable matters; that miasmatic diseases are endemic in places where the decomposition of organic matter is constantly taking place, as in marshy and moist localities; that they are developed epidemically under the same circumstances after inundations, and also in places where a large number of people are crowded together, with insufficient ventilation, as in ships, prisons, and besieged places.

Factitious fevers, produced by the introduction of deleterious substances directly into the blood, are analogous both in their symptoms and pathological lesions to those produced by the sting or bite of certain animals; they present also the same general class of symptoms that are present in smallpox, scarlatina, and other eruptive diseases. Putrid animal exhalations have given rise to diseases that have raged like a pestilence or epidemic. Measles can be communicated by means of a drop of blood from a patient affected with the disease; the inoculation of an unprotected person with smallpox may be the means of giving the disease to thousands; and a mere trace of serum is sufficient to propagate cattle plague. Recent researches into the peculiar nature and origin of the fever poison, or "disease germ," have not done much toward elucidating the question. Spectroscopic examination of the contagious fluids, variations of temperature, symptoms of the patient, anatomical alterations, and microscopic and chemical study of the blood upon the living and dead have furnished no notions sufficiently precise to draw any practical deductions. The most widely prevailing doctrine of the present day respecting the origin and communication of disease is that known as the germ theory.

Special organic forms known as mycrozymes, bacteria, bioplasts, etc, alleged by various pathologists to be found in contagious fluids, have been the subject of much discussion, some contending that they are of a fungoid growth and enter the body as parasites, others that they are germinal masses derived from normal cells, and due to a series of changes in existing matter under new circumstances; while a third class deny positively that any such germs exist. The elements or factors giving rise to many of the conditions above mentioned are known as zymotic, a term, like the atomic theory in chemistry, not clearly descriptive, but admitted into the standard nomenclature of medicine as a convenient expression, and including all that class of diseases which can be communicated from existing foci, and which are capable of being prevented by hygienic and other conditions. The latest and most approved nosology includes seven principal diseases of the zymotic class and eleven others less common: smallpox, measles, scarlet fever, diphtheria, croup, whooping cough, continued fever (including typhus, typhoid, and simple continued fever), quinsy, erysipelas, puerperal fever, carbuncle, influenza, dysentery, diarrhoea, cholera, ague, remittent fever, and rheumatism. - The reports of the registrar general of England show that more than one fifth of the whole number of deaths is from zymotic disorders.

An examination of the returns of the surgeon general and of the marine hospital bureau, as well as the health reports from the principal cities in the United States, will establish about the same ratio. This immense mortality, in view of the fact that all zymotic diseases are preventible, enforces the necessity of sanitary precautions.