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Bible Encyclopedias
Epidemiology
1911 Encyclopedia Britannica
or "==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 correlation 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, Hippocrates 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 constitution (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 crases, 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, Girolamo Fracastoro (1483-1553) clearly enunciated the principle of contagium vivum and, in the next generation, Guillaume Baillou (1538-1616) in his Epidenziorum 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 17th 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 greatest 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 18th century some annalists of sickness, especially the elder Wintringham (1689-1148), 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 particular 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 notably improved or that the opinions entertained by physicians at the beginning of the 19th century differed greatly from those of their predecessors.
During the first 30 years of the 19th century unrivalled opportunities 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, especially 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, particularly 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 common 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 10th century, and on the associated information which was obtained as to the causation and course of epidemics by careful local 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 which cause epidemics; on the conditions of living which favour the spread of infectious disease.
With the discovery of the organisms which cause disease and with the careful observation in the field as to the manner in which disease spreads from person to person, many new points of view have emerged. It is no longer sufficient to talk vaguely of fomites. Most diseases have their special forms of spreading which account for practically all the cases. Thus measles and smallpox are exceedingly infectious from person to person. Enteric fever is nearly always carried by contaminated water or contaminated food. Cholera is spread by water and flies. Other diseases have been found to be practically non-infectious from person to person unless by means of an intermediate parasite. Thus typhus and trench fever are carried by lice, while yellow fever and malaria require the intervention of the mosquito. The mode of spread of some diseases, however, is still obscure. Among these scarlet fever must be placed. While direct infection undoubtedly takes place a satisfactory elucidation of the problems of its dissemination has not yet been arrived at.
For accurate thinking on infectious diseases it must be noted that disease-producing organisms possess two qualities: one, the power of causing the disease, and the second the power of producing a severe attack of disease. The first may be termed infectivity and the second virulence. These qualities must not be confused. In point of fact they are not associated in any constant degree. Sometimes an epidemic begins with a large number of severe cases and sometimes the reverse. In certain diseases the height of the epidemic seems to be associated with severe disease, in others with that of milder type. The former at least holds for a certain number of large epidemics of measles of which the statistics have been investigated. The latter is the case both in Glasgow and London in regard to the autumnal prevalence of scarlet fever.
That an epidemic might possess a definite form capable of calculation seems to have been advanced first by Dr. Farr. In 1840 he graduated the decline of the great smallpox epidemic in England to the normal curve of error, and obtained a very close representation of the facts. He promised further discussion but seems to have given none till 1867. In this year he returned to the subject in connexion with the cattle plague, writing .a letter to the Daily News in which it was stated that though in the popular conception plague was advancing with such rapidity that all the cattle of the country might be destroyed, in reality the force of the epidemic was spent, and that if the form of the epidemic curve up to that point were taken as a basis of calculation the future course could be foretold. The prediction proved to be very near the truth.
The theory of the course of the epidemic, however, as a guide to the solution of the problem has unfortunately not proved so fertile as might have been hoped. Some facts are quite definite. The curve of the epidemic is generally found to be symmetrical, the fall corresponding closely to the rise, though in some diseases the ascent is more rapid than the descent, and in some the reverse. The equation of the curve which describes the majority of epidemics, as found by trial apart from theory, is _ a y (+)t' where y is the number of cases at time t, t being measured from the centre of the epidemic. Curves closely resembling that given by the above equation arise on a number of hypotheses of which two are discussed. First, the organism may be assumed to possess at the beginning of the disease a high degree of infectivity which decreases as the epidemic goes on. If the loss of infectivity is according to geometric law, the normal curve of error already used by Dr. Farr is the result. It is sufficient to state that on various probable hypotheses regarding exposure to infection, etc., the normal curve may be so modified as to take the form found by observation. Secondly, a similar type of curve arises if we consider an epidemic dies out from lack of susceptible persons. It is not possible to distinguish statistically these hypotheses from the consideration of the epidemic form alone. In one case, however, the second hypothesis can be tested. If the form of the epidemic be calculated by assuming different degrees of infectivity on the part of the organism, an infectivity which remains constant during the epidemic. it is found that this curve becomes flatter and flatter the smaller the degree of infectivity. Now with regard to plague in India among brown and black rats living more or less in the same circumstances, it is observed that man y more brown rats are infected than black. In such circumstances the form of the epizootic should be different in the two species if the decline is due to lack of susceptible individuals. As a matter of fact it is nearly identical: a fact which tells strongly. in favour of the hypothesis that the epidemic ends because of loss of infectivity on the part of the organisms. This example would be crucial but for the fact that the flea on which the spread of the epizootic depends has a law of seasonal prevalence of its own to which both the epizovtics must conform. In many cases, however, the only feasible explanation of the course of an epidemic is that the organism loses the power of infecting as the epidemic proceeds. It is impossible to suppose, for instance, with regard to the great epidemic of smallpox in London in 1901-2 that there were only 8,000 people susceptible, out of a population of 6,000,000. As the course of this epidemic was typical, rising and falling in the manner found to be characteristic, it cannot be argued that the decline was due to the action of the health authorities; all they can have done is to limit the extent of the epidemic, leaving its course unchanged. It is clear, therefore, that in circumstances like this there is some biological factor at work as distinct from a statistical factor. It may then be taken that epidemics in general have a particular form which is identical in many different diseases: plague, influenza, scarlet fever, etc. Even great differences of time do not bring about much change, the form of the epidemic of plague in Sydney in 1900 being nearly identical with that in London in 1665.
The next point requiring consideration is the periodicity in the epidemics of infectious diseases. Taking measles as an example, the common explanation is that each epidemic ends from the exhaustion of the number of susceptible persons, and that it is only when a new population of susceptible children has accumulated that a further outbreak occurs. This explanation fails to account for many of the facts. Even after the very large epidemic of measles in Glasgow in 1906, it was found that nearly half of the children admitted to the fever hospitals immediately thereafter suffering from other diseases had not suffered from measles so that there must have been, with the high infectivity of the epidemic, plenty of susceptible material. The disease subject to the most extensive inquiry hitherto has been measles. Using the method of the periodogram the statistics of London and all the chief towns of the British Isles have been analyzed. It is found that in almost no case is there only one period to be discovered. In London there are several, the chief of which is 97 weeks. This periodicity is found over the whole city. If the application of this mathematical method of analysis be admitted, this coexistence of epidemics of different. periods, each appearing at its own time, seems to prove that the termination of an outbreak of the disease is due to loss of infectivity on the part of the organism. Periodicity in other diseases is well known. Thus in the city of Liverpool the epidemics of scarlet fever occurred at regular intervals of four years from 1850-78. On one occasion alone was there an exception when the interval between two epidemics was three years in place of four. A similar periodicity of five years has been observed in Glasgow. There is one specially interesting example, namely the occurrence of plague in Bombay. In many places, such as Hong-Kong, the period between each epidemic is rigidly a year. In such a case the influence of the season of the year seems a sufficient explanation. But the case of Bombay is different. The first epidemic in 1897 had its maximum about the 40th day of the year. From this point until the last year for which statistics are available (1918), the date of the maximum of the epidemic has steadily advanced into the year, advancing about 80 days in 20 years or an average four days a year. It is difficult to account for a phenomenon like this except as being due to some property of the organism. The conclusion must be arrived at that while some periodicities of disease are strictly seasonal, others are not so, and require some further explanation.
A further important application of mathematics to epidemiology has been made by Sir Ronald Ross in his studies on malaria. Here the factors influencing the spread of the disease are numerous. Rainfall and temperature, the number of persons carrying the organism in their blood, and the number of mosquitoes and the proximity of the breeding-places of the mosquito to the abodes of men are all capable of quantitative measurement, and of furnishing guidance in the adoption of suitable administrative measures.
Climate and Weather
The relationship of epidemics to climate has received much attention in recent years, though in many cases the cause of seasonal prevalence is elusive. Thus why scarlet fever should be so regularly an autumnal disease is not at all clear. On many cases, however, much light has been thrown. The discovery, for instance, that malaria was carried by the mosquito elucidates the seasonal distribution of that disease.
A temperature of a certain height with associated pools of water is necessary for the rapid development of the mosquito and also a certain degree of temperature for the development of the parasite in the mosquito. In the same way the zone to which sleeping sickness is limited is a narrow region in which the climate and environment are suitable to the life history of one particular tsetse fly. Much light has been thrown on the epidemiology of plague by the discovery that it was carried to man from the rat by means of the flea. Humidity is necessary for the growth of the flea, and consequently epidemics of plague can hardly occur at seasons of the year when it is warm and dry. Thus the epidemics of plague in Bombay which have advanced progressively later and later into the year now occur when the flea is no longer at its greatest prevalence. With this change the number of cases and deaths has greatly diminished.
The epidemics of summer diarrhoea are also obviously climatic. The organism which causes the epidemic has not yet been discovered, but there is definite evidence that the amount of the disease is very closely associated with the summer temperature. When in London the weekly average of the air temperature rises above 60 F. and remains above that limit a large mortality is the result. Some evidence exists associating the occurrence of the disease with the presence of the house-fly, the fly carrying putrefying organisms from the garbage on which it feeds; but the presence of the fly and of diarrhoea at the same time does not prove that they are cause and effect. Both may well be abundant purely as, or the result of, a coincidence, the climatic conditions favouring both in an almost equal measure. A more difficult problem is the relation of weather to such infective diseases as the pneumonia of childhood. This disease is clearly associated with the winter season of the year but it does not seem specially affected by any special class of weather in that season. In the present state of knowledge it is in those diseases which depend on the spread of the organism by means of parasites that the most close association with weather has been made out.
Ef f ect of Organisms
We now come to the question on the relation of epidemics to the organism which causes them. Why an organism should be capable at one time of causing a great epidemic and at another only a few sporadic cases of a disease has not yet been found out. That organisms do vary in the power of infecting in this manner is a truism to anyone who has administered in the health departments of a large city. At one time the merest contact with a case of smallpox, for instance, will give rise to a large number of cases. At another time a patient suffering from smallpox may even attend in the gallery of a theatre without giving rise to a case of infection.
In recent years, a considerable amount of evidence has accumulated that an organism having found a suitable host or succession of hosts may have its virulence unusually exalted, and if the virulence can be exalted in this manner it is probable that some similar conditions may give rise to a great increase in the power of infection. At any rate, there is no doubt that in certain conditions organisms become highly infective and even the best sanitary precautions exercised in such circumstances can do little more than limit the amount of the disease. But there are further considerations which arise. It would seem as if at times two series of epidemics may coincide and may even mutually influence one another so as to produce a profound joint effect. Thus the great epidemic of influenza in the autumn of 1918 was associated with great activity of other pneumonia-producing organisms, the result being that the death-rate was of extreme amount and was distributed with age in a manner not found in any recent epidemic of influenza.
Environment
While an epidemic may in many cases be chiefly or even wholly due to the active condition of the causal organism it is to be remembered that the vitality and environment of the persons affected must also play a part. Thus, for instance, typhus fever introduced into a crowded slum in which lice are plentiful will almost certainly cause considerable havoc, but even here the havoc will be determined to a certain extent by the season of the year. If the weather be cold the people are crowded together on account of the demand for warmth, and the chance of infection is increased. In addition, in the winter food is often scarce and consequently vitality is low. If on the other hand the invasion of the organism takes place during the summer a large epidemic will be unlikely. But though these factors act, yet if an organism has an exalted state of activity, an epidemic of the disease may occur at any season of the year, even the most unlikely. Plague, for instance, especially in temperate climates, is essentially a disease of the warmer part of the year, yet it has been known occasionally to occur in large epidemics in the middle of winter, while epidemics of typhus of considerable size have been recorded in the summer time. The great epidemic of influenza in the autumn of 1918 is a marked example, such a season being in the extreme degree a very unusual one for an outbreak of this disease. What part special susceptibility on the part of the population, due to change in vitality, played in this case is not known. Some other influences also act. There is some evidence that fatigue predisposes to enteric fever, an army on the march drinking polluted water tending to have a larger number stricken than a similarly conditioned civil population. Further, it cannot be doubted that the accumulated effect of seasons may tend to depress health and increase susceptibility to certain diseases. The cumulative effect of winter cold may be perhaps traced in children in relation to death from whooping-cough, the average minimum temperature in the winter preceding the maximum number of deaths from whooping-cough by about six weeks, while the form of the two curves is very much the same. The deaths from whooping-cough are due very largely to broncho-pneumonia, yet the seasonal distribution of whooping-cough is not identical with that of the latter disease. Thus scarlet fever, being an autumnal disease and following the hot summer, might in the same way be ascribed to depression produced by continued hot weather, making certain persons more susceptible to the disease. But as scarlet fever is a disease almost absent in warm climates this explanation can hardly be complete, and some other factor must be necessary. None of these questions, however, have at present been sufficiently investigated to allow any dogmatism.
Another point of importance requires special reference, and that is the problem of " carriers," as individuals infected with a disease and cured as regards themselves, but who yet continue to harbour and distribute the parasite, are called. Cholera follows the pilgrims' way, enteric fever the carrier cook, diphtheria the carrier school-teacher.
References
The most important of the epidemiological writings of Hippocrates are the Epidemics (Books 1 and 3) and the treatise on Airs, Waters and Places, both included in the Sydenham Society's translation (by Francis Adams) and in Littre's text (with French translation). Galen's most important works are De Febrium Differentiis and his comments on the Hippocratic Epidemics (both in Kuehn's edition with Latin translation). The best edition of Sydenham is that edited for the Sydenham Society by Greenhill. An excellent general account of the progress of knowledge is contained in Haeser's Lehrbuch der Geschichte der Medizin and der epidemischen Krankheiten, 3 vols. 3d ed. (1882). English epidemiological history is fully related in Dr. Charles Creighton's History of Epidemics in Britain, 2 vols. (1894).
Two papers by Greenwood on the " Epidemiology of Plague in India," Journal of Hygiene, vol. x. p. 349 and vol. xi. p. 62, give examples of modern epidemiological method, while his Report " On the Rise, Spread, etc., of Epidemic Diseases," Internat. Congress of Medicine, Sec. xviii., London 1913, gives a full study with literature. Two papers by John Brownlee discussing " Theory of Epidemiology ' in Relation to Plague " (Proc. Roy. Soc. Med. 1918, vol. xi., p. 86) and the " Periodicities of Epidemics of Measles " (Proc. Roy. Soc. Med. 1919, vol. xii., p. 77) give an account of the statistical and mathematical methods which may be used. Ross's Prevention of Malaria and Boyce's Yellow Fever and its Prevention discuss theory and practice in all their forms. O. BRO.; M. G.*)
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Chisholm, Hugh, General Editor. Entry for 'Epidemiology'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/e/epidemiology.html. 1910.
