[Footnote 63: The terms "contagion" and "contagious" are here used in their widest signification, and are applied in this essay to all diseases capable of propagation by infected individuals to persons in health.]
It is still a vexed question whether a disease that is capable of self-propagation can ever be generated de novo. It is maintained, on the one hand, that such an occurrence is as impossible as the spontaneous generation of plants or animals; while, on the other hand, it is argued that the poison of certain diseases capable of self-propagation may, under certain favorable conditions, be produced independently of any pre-existing cases of the disease. The comparison of a fever-poison with a spore or ovum is an ingenious, but a most delusive, argument. An epidemic disease springing up in a locality where it was before unknown, and where it is impossible to trace its introduction from without, is said to be not more extraordinary than the development of fungi in a putrid fluid. The argument, however, is founded on a pure assumption, for there is not a tittle of evidence to show that a fever-poison is of the nature of a spore or ovum. Air saturated with the poisons of various contagious diseases has been condensed and submitted to the highest powers of the microscope, but nothing approaching to a small-pox spore, or a typhus ovum, has yet been discovered. It is true that certain contagious diseases, such as scarlet-fever and smallpox, can in most instances be traced to contagion; but, with regard to others, such as typhoid or enteric fever, it is in most instances utterly impossible to account for the first cases in any outbreak on the theory of contagion, while, at the same time, there is direct evidence that the contagious power of the disease is extremely. The question is no doubt beset with many difficulties, and constitutes one of the most intricate problems in medical science. It is one, however, which can never be solved by entering on the discussion with a preconceived theory as to the close analogy, if not identity, of a fever-poison with an animal or vegetable ovum, nor by assuming that the laws which regulate the propagation of one contagious disease are equally applicable to all. Nature's facts are too often interpreted by human laws, rather than by the laws of nature. In the case before us, the natural history of each disease must be studied independently, and our ideas as to its origin and mode of propagation must be founded on the evidence furnished by that study alone, and irrespective of the laws which seem to regulate the origin and propagation of other diseases with which it has no connection whatever, except in the human mind. At the present moment, when the subject of epidemics is attracting so much attention, it may be interesting to call attention to the more important diseases comprised under that head, and to point out some of the main facts connected with their origin and distribution. [{422}] The principal epidemic diseases, then, are: small-pox, scarlet-fever, measles, typhus, relapsing fever, Oriental plague, yellow fever, diarrhoea, typhoid or enteric fever, cholera, dysentery, ague and remittent fevers, influenza, the sweating sickness, and the dancing mania.
1. Small-pox the most loathsome of all diseases, is believed to have prevailed in India and China from time immemorial. About the middle of the sixth century it is supposed to have been conveyed by trading vessels from India to Arabia, and the Arabian army at the siege of Mecca, in the year 569, was the first victim of its fury. From Arabia it was imported into Europe by the Saracens, and there is evidence of its existence in Britain before the ninth century. Before the introduction of vaccination, small-pox was one of the chief causes of mortality in all the countries where it prevailed, and even now it occupies a prominent place in our mortuary returns. During the twenty-four years 1838-61, 125,352 of the population of England and Wales, and 21,369 of the population of London, died of small-pox; or, in other words, one in seventy-five of the total deaths in England and Wales, and one in sixty-three of the total deaths in London, were due to this disease. Small-pox is not confined to any race or quarter of the globe. At the present day its appearance can, in the great majority of instances, be traced to contagion. It is evident, however, that it must at one time have had an origin, and it is reasonable to infer that what happened once may happen again. Small-pox is known to attack many of the lower animals as well as man, and there are grounds for believing that it originated among the former, and by them was communicated to the human species. A careful study of epizootics—our ignorance of which has been disclosed by the present cattle plague —may ultimately reveal the mode of origin of the poison of small-pox. The disease varies greatly in its prevalence at different times. In other words, it is sometimes epidemic, at others not. Some of these epidemics are local; others are widely extended. All exhibit a gradual rise, culmination, and decline, the decline being always less rapid than the advance. It is difficult to account for the occurrence of these epidemics. They are independent of hygienic defects, season, temperature, or any meteorological conditions of which we are cognizant. They are probably due to causes tending to depress the general health of the population, and so to predispose it to the action of the poison. For nearly two centuries it has been a common observation that epidemics of small-pox have co-existed with epidemics of other contagious diseases. The gradual accumulation also in a district of unprotected persons, owing to the neglect of vaccination, will also predispose to the occurrence of an epidemic, after the introduction of the poison. In fact, to the neglect, or careless performance, of vaccination, is entirely due the occurrence of epidemics of small-pox at the present day.
2. Scarlet Fever. —The early history of scarlet fever is obscure, for the disease was long confounded with measles and small-pox, but it is generally supposed that, like small-pox, it came originally from Africa, and was imported into Europe by the Saracens. It has been known to prevail in Britain for the last two centuries; but although it is only of late years, from the reports of the Registrar-General, that we have been able to form an accurate idea of the extent of its prevalence, there can be no doubt that it has increased greatly during the present century, and that it now occupies that pre-eminence among the causes of mortality in childhood which was formerly held by small-pox. During twenty-four years (1838 to 1861 inclusive) 375,009 of the population of England and Wales, and 58,663 of the inhabitants of London, died of scarlet fever, or about one in every twenty-four deaths that occurred in England during the period in question [{423}] was due to this disease. The mortality from scarlet fever, in fact, exceeds the mortality from small-pox and measles taken together. Scarlet fever is known to prevail over the whole of the continents of Europe and America, but it is nowhere so common as in Britain. In France it is a rarer disease than either measles or small-pox. In India it is said never to occur. In most instances it is not difficult to trace the occurrence of scarlet fever to contagion; and from the remarkable indestructibility of the poison and its tendency to adhere to clothes, furniture, and even to the walls of houses, there can be little doubt that the disease has a similar origin in many instances, where the mode of transmission of the poison cannot be traced. How the poison first originated is yet a mystery; but there is some probability in the view, which has many able advocates, that it originated in horses or cattle, and by them was communicated to man. If this be so, it is reasonable to hope that investigations as to the occurrence of the disease in the lower animals may lead to a discovery productive of as great benefits to the human race as vaccination. At intervals of a few years scarlet fever spreads as an epidemic: but its ordinary prevalence, in this country is greater than is generally imagined. The causes of these epidemic outbursts are unknown. Many circumscribed outbreaks can no doubt be traced to the importation of the poison into a population of persons unprotected by a previous attack; but why the poison should be introduced into numerous localities at one time, and not at others, is difficult to determine. It is tolerably certain, however, that at all times the prevalence of the disease is independent of overcrowding, bad drainage, or of any appreciable hygienic or meteorological conditions.
3. Measles was long confounded with scarlet fever, and, like it, is supposed to have been originally imported from the East. During twenty-four years (1838-1861) this disease destroyed 31,595 of the population of London, and 181,868 persons in England and Wales. It is known to occur in all parts of the world, and is highly contagious. There is no evidence that any hygienic defects or meteorological conditions can generate the poison of measles. Hildenbrand, a great authority, thought it might arise where numbers of men and cattle were confined together in close, unventilated buildings; and in later times American and Irish physicians have described a disease corresponding in every respect with the measles, which appeared to arise from sleeping on old musty straw, or from the inoculation of the fungi of wheat straw. Measles in England is much less of an epidemic disease than either small-pox or scarlet fever. The number of deaths which it causes in years when it is most prevalent, is rarely much more than double what it causes in years when it is at least prevalent. Although often most fatal in winter, there is no proof that its prevalence is influenced by season.
4. Typhus Fever has been well known for upward of three centuries, and there are grounds for believing that from remote ages it has prevailed in most parts of the world under favorable conditions. It is impossible to estimate the precise extent of its prevalence, inasmuch as many other diseases are included under the designation "typhus," in the reports of the Registrar-General; but it is the acknowledged scourge of the poor inhabitants of our large towns. There is no evidence that typhus, such as we see it in this country, has as yet been observed in Australia, New Zealand, Asia, Africa, or the tropical parts of America. Even in Britain it is confined, for the most part, to the large towns, and to the poorest and most densely crowded parts of them. It is a disease almost unknown among the better classes, except in the case of clergymen and doctors who visit the infected poor. [{424}] It is undoubtedly contagious; but in a spacious dwelling with a free ventilation, it almost ceases to be so. There is also ample evidence that the poison may be generated de novo; and the circumstances under which this occurs are overcrowding, with defective ventilation and destitution. Hence it is that the disease was formerly so apt to show itself in prisons and ships, and that, even at the present day, it is so common an attendant on warfare and so prevalent in the wretched hovels of the poor. This was the disease that before the days of Howard was never absent from our prisons and hospitals, and that decimated the armies of the first Napoleon and of the allies in the Crimea. "If," says an able writer on fever in the last century, "any person will take the trouble to stand in the sun, and look at his own shadow on a white plastered wall, he will easily perceive that his whole body is a smoking dunghill, with a vapor exhaling from every part of it. This vapor is subtle, acrid, and offensive to the smell; if retained in the body, it becomes morbid; but if re-absorbed, highly deleterious. If a number of persons, therefore, are long confined in any close place not properly ventilated, so as to inspire and swallow with their spittle the vapors of each other, they must soon feel its bad effects. Bad provisions and gloomy thoughts will add to their misery, and soon breed the seminium of a pestilential fever, dangerous not only to themselves, but also to every person who visits them or even communicates with them at second-hand. Hence it is so frequently bred in gaols, hospitals, ships, camps, and besieged towns. A seminium once produced is easily spread by contagion." But if overcrowding produces typhus, why is it that the disease prevails in the epidemic form, and then in a great measure disappears? The explanation is in this way. All the great epidemics of typhus have occurred during seasons of famine or of unusual destitution. One of the most common consequences of general destitution is the congregation of several families in one house, in consequence of their inability to pay their rents, and of the concentration in the large towns of many of the inhabitants of country districts. Famine pre-disposes to typhus by weakening the constitution; and it also tends to produce it, in so far as it causes an unusual degree of overcrowding. It has been the custom with many writers to refer epidemics of typhus to some subtle "epidemic influence;" and thus, where a failure of the crops has been followed by typhus, both of these disasters have been ascribed to a common atmospheric cause. But of such atmospheric influences, capable of producing typhus, we know nothing; their very existence is doubtful, and the employment of the term has too often had the effect of cloaking human ignorance, or of stifling the search after truth. If typhus be due to any "epidemic influence," why does this influence select large towns and spare the country districts? why does it fall upon large towns in exact proportion to the degree of privation and overcrowding among the poor? in large towns, why does it indict the crowded dwellings of the poor and spare the habitations of the rich? and why did the varying prevalence of typhus among the French and English troops in the Crimea correspond, exactly to the varying degree of overcrowding in either army? Moreover, famine artificially induced by warfare, by commercial failures, by strikes, or by any cause that throws large bodies of men out of employment, is equally efficacious in originating epidemics of typhus, as famine from failure of the crops.
5. Relapsing Fever is so called from the fact that after a week's illness there is an interval of good health for a week, followed by a second attack. It is contagious, and is epidemic in a stricter sense than even typhus. Although sometimes more prevalent in this country than any other fever, it may disappear for so many years that [{425}] on its return it has more than once been thought to be a new malady. For upwards of ten years not a case of it has been observed in Britain, but it has constitute the chief component of many of the greatest epidemics of fever which has devastated this country and Ireland, and it was one of the diseases composing the "Russian Plague," which in the spring of the present year caused such unnecessary alarm in this country. It usually prevails in the epidemic form in conjunction with typhus, and it is connected in its origin more directly with protracted starvation and the use of unwholesome food than even the latter disease. Hence, in this country, it is familiarly known as "Famine Fever," and in Germany as "Hungerpest. "
6. Oriental Plague is still met with in Egypt and in other eastern countries; but in the middle ages it frequently overran the whole of Europe and invaded England, and, from the extent of its ravages, it was known as the "Black Death," and the "Great Mortality. " The Great Plague of London, of 1665, is a familiar fact in history. Since then the disease has not been met with in this country. But British typhus is merely a modified form of Oriental plague, or, in other words, plague is merely typhus complicated with numerous abscesses beneath the skin. Cases of typhus are occasionally met with in this country, corresponding in every respect with true plague. Both diseases appear under similar circumstances, but those which generate plague are of a more aggravated character than those which suffice to produce typhus. The disappearance of plague from London, notwithstanding our vastly increased communications with Egypt, has been chiefly due to the better construction of our dwellings since the "Great Fire" of 1666. "It is probable," says an able writer on the plague, "that if this country has been so long forsaken by the plague as almost to have forgotten, or at least to be unwilling to own, its natural offspring, it has been because the parent has been disgusted with the circumstances under which that hateful birth was brought to light, has removed the filth from her doors in which it was matured, and has adopted a system of cleanliness fatal to its nourishment at home. But if ever this favored country, now grown wise by experience, should relapse into former errors, and recur to her odious habits, as in past ages, it is not to be doubted that a mutual recognition will take place, and she will again be visited by her abandoned child, who has been wandering a fugitive among kindred associates, sometimes in the mud cots of Egypt, sometimes in the crowded tents of Barbary, and sometimes in the filthy kaisarias of Aleppo."
7. Yellow Fever is a contagious fever with a limited geographical range. Its geographical limits, as regards the new world, are from about 43° N. lat. to 35° S. lat; and in the old world from 44° N. to 8° or 9° S. lat. It is a common disease on board our ships stationed in the West Indies and off the west coast of Africa. As in the case of typhus, overcrowded and defective ventilation are the main causes which favor its origin and propagation, and, indeed, it is still a subject for investigation whether yellow fever may not be typhus modified by climate and other circumstances. One of the most recent and best authorities [Footnote 64] on the disease thus writes: "Overcrowding in the between-decks of steamships seems to be the principal cause of the extreme fatality of the disease in the navy. What in this respect is true of typhus may with equal force be said of yellow fever. There is no such powerful adjuvant to the virulence of the poison, and to its power of propagation, as an unrenewed atmosphere, loaded with human exhalations."
[Footnote 64: Dr. Gavin Milroy, President of the Epidemiological Society.]