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 in adventures in Asia Minor. He was reported in 1920–21 to have been employed at Moscow	 as director of the Asiatic department in the Soviet Government, and to have posed at the Baku Congress of Oriental Peoples as the leader of a great Socialist movement in the middle east and north Africa. EPIDEMIOLOGY. In recent years more study has been given to that branch of the science of medicine which, under the name of epidemiology, displays the general factors which operate upon populations or aggregates and lead to the outbreak of a sickness affecting several persons within a short interval of time. The unit of the epidemiologist is a population, while the unit of a physician is an individual.

The first scientific epidemiologist was Hippocrates, whose treatises On Epidemics and On Airs, Waters and Places remain models of epidemiological inquiry. In the latter work, he displayed the cor- relation between the physique, habits of life and climatological advantages or disadvantages of various populations and the types of illness prevalent amongst them. In the former, by means of an intensive study of the diseases prevailing through a series of years in one and the same place, he established the conception of an epidemiological type or constitution determined to a greater or less degree by meteorological conditions. Incidentally Hippocrates described some forms of epidemic disease, such as mumps, in terms fully applicable to modern experience. He also recognized the tendency of particular types of epidemic sickness to appear at a change of season, especially near the vernal or autumnal equinox. In treating of disease as a mass phenomenon, of epidemics, Hippoc- rates exhibited the scientific caution and zeal for the collection of objective data upon which to found an induction which have rendered immortal his clinical studies. Galen, whose authority for many centuries overshadowed that of the founder of Greek scientific medicine, systematized the theoretical teaching of Hippocrates but recorded few fresh observations. According to Galen, a disease was a function of three variables: the innate or acquired constitu- tion (crasis or temperament) of the body, disordered habits of life, atmospheric changes (metastases). Illness became epidemic when, some abnormal modification of the atmosphere having occurred, the temperaments, or erases, of a sufficient number of the persons exposed were apt to give rise to illness. He recognized the contagious nature of certain diseases, such as ophthalmia and phthisis, but, in his terminology, contagion was very different from what we now understand by it. He had no notion of a vital infective principle, a contagium vivum, but looked upon the transmission of disease from person to person more as one now looks upon the setting into vibration of a series of tuning-forks when their fundamental notes are struck. None of the post -Galenical or Greek physicians or of the Arabian writers added much to our practical knowledge of epidemiology. In the 16th century, Girolamp Fracastoro (1483–1553) clearly enunciated the principle of contagium vivum and, in the next generation, Guillaume Baillou (1538–1616) in his Epidemiorum et ephemeridum libri II. (first printed in 1640) resumed the plan of actually describing the forms of illness prevalent in successive years which was the foundation of Hippocratic epidemiology.

Neither the importance of Fracastoro's principle nor the value of the method originated by Hippocrates and adopted by Baillou were realized by contemporary physicians, and, although accurate description of particular outbreaks accumulated during the IJth century, a general science of epidemiology was still to seek.

The honour of being the second founder of scientific epidemiology is usually assigned to Thomas Sydenham, and although this physician had no notion of the importance of Fracastoro's ideas and in his adoption of the Hippocratic plan had been anticipated by Baillou, the attribution is just.

To Sydenham (1624–89) belongs the credit of having realized that the succession of diseases is not chaotic and of having attempted to deduce from personal observations extended over more than 20 years a general doctrine of epidemiology. Sydenham's observations are not always clearly recorded, nor were his conclusions entirely free from inconsistencies, but his main principles were the following. He thought that all types of disease prevalent at any one time bore the imprint of a common " constitution " the ultimate source of which he supposed to be indefinable telluric variations the overt expression of the constitution was a " stationary fever," found in different clinical settings. Hence two different specific " diseases " prevailing during one " constitution " resembled one another more closely than did instances of the same " disease " observed under two different " constitutions." To this distinction he attached the great- est importance as a practitioner of medicine: " This only, fortified by a multitude of exact observations, I do confidently hold, that the aforesaid species of disease, in particular the continued fevers, may vary so enormously that you may kill your patient at the end of the year by the method which cured sufferers at the beginning of it."

Sydenham classified his successive “constitutions” in accordance with the clinical form of illness most usually observed under it and closely watched the changes of symptomatic form which heralded the emergence of a new " constitution."

Although in modern times this notion of an epidemiological succession has been a fruitful hypothesis and many of Sydenham's predictions as to the decline of reigning diseases and their replacement by others have been accurately fulfilled, his immediate influence upon epidemiological thought was much less effective than his moulding of clinical practice. The reason is that to sift the wheat from the chaff of his ideas required a new instrument, viz. a statistical method applied to numerical data. Neither method nor adequate data existed at the end of the 17th century. The science of epidemiology owes almost as much to Sydenham's contemporary, the London draper John Graunt (1620–74), who founded vital statistics, as to the English Hippocrates. During the i8th century some annalists of sickness, especially the elder Wintringham (1689–1748), Huxham (1692–1768), Van Swieten (1700–72) and Anton Storck (1731–1803), provided more data on the Hippocratic model, and practical contributions to the art of hygiene and the control of par- ticular epidemics were made by such investigators as Lind (1716–94), Pringle (1707–82), Monroe (1727–1802), Brocklesby (1722–97) and Blane (1749–1834). Contemporaneously, a series of illustrious mathematicians, from Pascal to Laplace, were forging the instruments of statistical research which in the hands of Farr were destined to render great advances in scientific epidemiology possible. It cannot, however, be said that the general doctrines of epidemiology were not- ably improved or that the opinions entertained by physicians at the beginning of the igth century differed greatly from those of their predecessors.

During the first 30 years of the ipth century unrivalled op- portunities were afforded for the study of particular epidemic diseases, especially typhus and typhoid, owing to the Napoleonic Wars and the industrial revolution with its attendant social disorganization. A new interest in public health matters, especial- ly in England, led to the accumulation of facts respecting the circumstances attending the outbreak of epidemic diseases. Before the establishment of the English General Register office (in 1837) official reports upon epidemiological matters, particu- larly cholera, had been furnished and the ground prepared for the work soon to be undertaken by William Farr (1807–83).

Broadly speaking, the state of epidemiological knowledge at the beginning of the reign of Victoria was as follows. The contagious nature of the diseases known as zymotics was fully recognized and the specific difference between scarlet fever and diphtheria understood. The relation between pollution of water supplies, cholera and certain other forms of “continued fever” with intestinal lesions had also been perceived. Experience of vaccination had firmly established a belief in the possibility of immunizing mankind against one form of epidemic disease. At least one physician, Robert Watt, of Glasgow (1774–1819), had contributed new evidence of a statistical character in favour of Sydenham's doctrine of epidemiological succession, while the remarkable increase of malignity which began to characterize scarlet fever during the third decade of the century and the return of pandemic influenza (a disease described by many writers in and before the 18th century) had impressed the same ideas upon the general body of the medical profession. On the other hand, the fundamental distinction between typhoid and typhus fever and the epidemiological importance of the distinction had only been realized by a few exceptional men, and statistical data necessary for the assessment of the epidemiological factors com- mon to groups of diseases and for the testing of epidemiological theories were fragmentary.

The Spread of Epidemics. Modern epidemiology is based on the collections of statistics which began half way through the igth century, and on the associated information which was ob- tained as to the causation and course of epidemics by careful lo- cal inquiry into all the conditions. It is true that before this some countries, such as Sweden, had published the figures of the deaths from numerous infectious diseases for series of years, but though these figures are very interesting they represent more or less special conditions. Since about 1840, especially in Europe, in India and America, carefully collected information exists respecting many epidemics and epidemiological conditions. Sufficient evidence is now available to examine any theory which may be offered to account for the facts. Advance has been made on a number of lines: on the modes of spread of infection; on the theory of the course, recurrence, and size of epidemics; on the relation of epidemics to climatic conditions and the cause of these relations; on the knowledge of the life history of the organisms