Introduced by travellers, or spreading from tribe to tribe, they appeared suddenly in epidemic form as plagues and pestilences, and, disappearing as suddenly, were not known again till a fresh generation furnished a fresh supply of food.
When, however, in spite of war, famine, and pestilence, the human race increased to such an extent that the number of fresh births furnished a perennial supply of food, while at the same time a rising civilisation and improved means of communication lessened the isolation of various communities, then many diseases slowly passed from an epidemic to an endemic form. Pestilence grew rare, but every individual was exposed to infection, and, during youth, either perished from, or acquired immunity against, the more prevalent forms of disease.
Measles a National Scourge
When endemic, zymotic disease—at any rate, disease against which immunity can be acquired—is far less terrible than when epidemic. Modern examples of ancient epidemics may be seen in isolated regions. In Pacific islands, for example, air-borne disease spreads like a flame. The whole community is stricken down. The sick are left untended and perish in multitudes. The entire business of the community is neglected, and famine frequently follows. Under such conditions measles or whooping-cough, diseases which we in England are accustomed to regard as scarcely more than nuisances, may rise to the level of a great national disaster. Thus, in 1749, 30,000 natives perished of measles on the banks of the Amazon. In 1829 half the population died in Astoria. In 1846 measles committed frightful ravages in the Hudson Bay territory. More recently a quarter of the total inhabitants was swept away in the Fiji group of islands.
Sanitation is Sometimes Powerless
At the dawn of history, long after the evolution of zymotic disease, the population of the Eastern Hemisphere was still sparse and scattered. Even as late as the Norman Conquest that of England was barely two millions—about one-third of the number now present in London. Means of communication were poor and beset by dangers. A journey from York to London was then a more serious affair than a journey from London to San Francisco to-day. Water and air borne diseases were, therefore, absent during long periods of time. When they came they spread as epidemics. Accordingly we read of plague and pestilence; of diseases suddenly becoming epidemic and sweeping away a fourth or half of entire communities. Historians are apt to attribute these immense catastrophes partly to the bad sanitation of the period and partly to diseases which have died out of the world, or, at any rate, out of Europe. Doubtless they are right in a few instances. But, apart from diseases which spread under special circumstances from tropical centres, had sanitation, under modern conditions of intercommunication and crowding, tends to render water-borne disease endemic, not epidemic. Over air-borne disease it has no effect. Measles, whooping-cough, chicken-pox, influenza, common cold, and small-pox (in a modified form) are as common as ever.
Plagues “the Wrath of God”
The character of these ancient epidemics, their special symptoms as indicated in old literature, their sudden and portentous appearance, which men attributed to the wrath of God, their tremendous infectivity and rapid spread, their equally sudden and complete departure as of Divine anger assuaged, point rather to air and water borne diseases of the types now endemic and comparatively harmless among us, but still so fearful in their effects on isolated communities. Like the light flashed from a child’s mirror on a darkened wall, so they flickered and swept forwards and backwards from end to end of the Old World—from the Malay Peninsula to the North Cape of Norway, from Kamschatka to the south point of Africa. A parallel may be found in the recent epidemic of rinderpest amongst the herbivorous animals of Africa. Years might pass, old men might remember, the peoples might sacrifice to their gods; but when a fresh generation of those who knew not the disease had arisen, when the harvest of the non-immune was ripe and ready, the diseases would return to the dreadful reaping. Behind them the earth was heaped with the dead, and the few and stricken survivors grubbed for roots to satisfy their hunger. To-day sanitation has nearly abolished water-borne diseases, and, in a population largely immune, epidemics of air-borne disease, like a light thrown on a sunlit wall, are but faint shadows of that which they were in their old days of awful power.
Growth of Resisting Power
The progress of consumption was different; it was never truly epidemic. Owing to its low infectivity, to its lingering nature, to the fact that no immunity could be acquired against it, it did not spread suddenly when first introduced, but when once established its virulence did not abate within measurable time. In other words, it was endemic from the beginning. It made its home in the hovels of the early settlers on the land. In such situations—as in Polynesian villages—modern Englishmen do not take the disease. But their remote ancestors were more susceptible; they could be infected by a smaller dose of the bacilli. Gradually, as civilisation advanced, the conditions grew more stringent; men gathered into larger and denser communities, into hamlets and villages in which they built houses ill lighted and worse ventilated.