CityReads|Humanity’s Encounters with Infectious Diseases

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 Humanity’s Encounters with Infectious Diseases

Humanity is vulnerable to the infectious diseases.

William H. McNeill, 1976, Plagues and Peoples,1st Edition, Anchor

Upon its original publication, Plagues and Peoples was an immediate critical and popular success, offering a radically new interpretation of world history as seen through the extraordinary impact–political, demographic, ecological, and psychological–of disease on cultures. It has become a classic influencing many important thinkers, such as Jared Diamond. And it will change your worldviews. The ongoing Wuhan coronavirus outbreak has been declared by the WHO as a global health emergency. This book can help us better understand humanity’s vulnerability to the infectious diseases throughout history.

Human and parasites: a precarious equilibrium

Most human lives as caught in a precarious equilibrium between the microparasitism of disease organisms and the macroparasitism of large-bodied predators, chief among which have been other human beings.

Microparasites are tiny organisms—viruses, bacteria, or multi-celled creatures as the case may be—that find a source of food in human tissues suitable for sustaining their own vital processes. Some microparasites provoke acute disease and either kill their host after only a brief period of time, or provoke immunity reactions inside his body that kill them off instead. Sometimes, too, one of these disease-causing organisms is somehow contained within a particular host’s body so that he becomes a carrier, capable of infecting someone else without being noticeably sick himself. There are, however, other microparasites that regularly achieve more stable relations with their human hosts. Such infections no doubt take something away from their host’s bodily energies, but their presence does not prevent normal functioning.

Prolonged interaction between human host and infectious organism, carried on across many generations and among suitably numerous populations on each side, creates a pattern of mutual adaptation which allows both to survive. A disease organism that kills its host quickly creates a crisis for itself, since a new host must somehow be found often enough, and soon enough, to keep its own chain of generations going. Conversely, a human body that resists infection so completely that the would-be parasite cannot find any lodgment, obviously creates another kind of crisis of survival for the infectious organism.

In all such processes of adjustment, bacteria and viruses have the advantage of a much shorter time between generations. Genetic mutations that facilitate propagation of a disease organism safely from host to host are consequently able to establish themselves much more rapidly than any comparable alterations of human genetic endowment or bodily traits can occur. Indeed, historical experience of later ages suggests that something like 120 to 150 years are needed for human populations to stabilize their response to drastic new infections.

The process of adaptation between host and parasite is so rapid and changeable that we must assume that patterns of infection prevailing now are only the current manifestations of diseases that have in fact altered their behavior in far-reaching ways during historic times.

Human distribution across diverse climatic zones created what may be called a parasitic gradient among the different communities that resulted. The general thinning out of the variety of life forms that took place as climates became colder and/or drier implies, after all, a diminution in the number and variety of parasitic organisms capable of afflicting human beings. Conditions for successful transfer from host to host became more difficult as temperatures (and humidity) dropped and as the seasons of warmth and sunlight shortened. The effect was to create a gradient of infection and infestation such that populations from warmer, wetter climes could travel to cooler and/or drier regions with little likelihood of encountering unfamiliar parasites, whereas infections and infestations lurking in southern and warmer or wetter lands constituted a standing threat to intruders from the cooler North or drier desert.

Densities of human populations, the character and quality of available water supply, food, and shelter, together with the frequency and range of contacts among individuals all could affect disease patterns significantly. Great cities were, until recently, always unhealthy, even when situated in cool or dry climates. Generally speaking, though, all such local disturbances of ecological relations have worked within a biological gradient characterized by an increase in the variety and frequency of infections as temperatures and moisture increased.

For example, very potent obstacle stood in the way of the swift and successful development of rice paddies and urban life in lands to the south of the historic cradle of Chinese civilization: for in moving southward and into better farming regions, Chinese pioneers were also climbing a rather steep disease gradient! The warmer, moister condition of the South allowed a greater variety of parasites to flourish than could survive in the North. Populations accustomed to disease conditions of the North therefore faced formidable problems in adjusting to the markedly different patterns of parasitism that prevailed farther south. Thus, Ssu-ma Ch’ien, the founder of Chinese historiography, who lived from about 145 to 87 B.C., tells us: “In the area south of the Yangtse the land is low and the climate humid; adult males die young.”

Agriculture, sedentary living and the infectious diseases

Food production permitted a vast and rapid increase in the number of people, and soon sustained the rise of cities and civilizations. Human populations, once concentrated into such large communities, offered potential disease organisms a rich and accessible food supply.

Generally speaking, although the New World was remarkably impoverished in domesticable animals, it did have a number of useful plants, whereas the Old World offered human ingenuity both a wide range of domesticable animals and an impressive array of potential food plants.

Food chains shortened as human action reduced the roles of rival predators and reserved an increased amount of food for the consumption of a single species: Homo sapiens. Shortening natural food chains involved humankind in never-ending effort. Protecting herds and crops from animal predators was not a serious problem for skilled hunters, though it required perpetual vigilance. Protection from other men, however, was a different matter, and efforts to achieve safety from human marauders provided the chief stimulus to political organization—a process by no means completed yet.

Shortening the food chain and multiplying a restricted number of domesticated species of plants and animals also created dense concentrations of potential food for parasites. Since most successful parasites were too small to be seen, for many centuries human intelligence could not cope very effectively with their ravages.

Settling down to prolonged or permanent occupancy of a single village site involved new risks of parasitic invasion. Human populations living in sedentary communities were therefore far more thickly infested with worms and similar parasites than their hunting predecessors or contemporaries in the same climatic zones.

But irrigation farming, especially in relatively warm climates, came near to recreating the favorable conditions for the transmission of disease parasites that prevailed in tropical rain forests whence humanity’s remote ancestors had presumably emerged. Abundant moisture—even more abundant than that commonly available in rain forest environments—facilitated transfer of parasites from host to host. Where suitably warm and shallow water, in which potential human hosts constantly waded about, provided a satisfactory transfer medium, parasites did not need resistant cysts, or other life forms that could withstand dry conditions for lengthy periods of time.

Protozoan, bacterial, and viral infections also had an expanded field for their propagation as flocks, crops, and human populations all multiplied.

In central and eastern Africa, events in the nineteenth and twentieth centuries connected with ill-conceived efforts by European colonial administrators to alter traditional patterns of herding and cultivation also illustrate the unexpected side effects that sometimes arise from agricultural expansions into new regions. These efforts, in fact, precipitated veritable epidemics of sleeping sickness in parts of Uganda, the Belgian Congo, Tanganyika, Rhodesia, and Nigeria; and the end result, as colonial regimes came to an end, was a land more thickly infested with death-dealing tsetse flies than before government policy set out to utilize what looked like good agricultural land more effectively.

Obviously human attempts to shorten the food chain within the toughest and most variegated of all natural ecosystems of the earth, the tropical rain forests and adjacent savanna regions of Africa, are still imperfectly successful, and continue to involve exceptionally high costs in the form of exposure to disease. That, more than anything else, is why Africa remained backward in the development of civilization when compared to temperate lands (or tropical zones like those of the Americas).

City and the infectious diseases

Person to person, “civilized” types of infectious disease could not have established themselves much before  3000  B.C.  When  they  did  get  going,  however,  different  infections  established  themselves  among different  civilized  communities  in  Eurasia. Proof  of  this  fact  is  that  when  communications  between previously isolated civilized communities became regular and organized, just before and after the Christian era, devastating infections soon spread from one civilization to another.

Only in communities of several thousand persons, where encounters with others attain sufficient frequency to allow infection to spread unceasingly from one individual to another, can such diseases persist. These communities are what we call civilized: large, complexly organized, densely populated, and without exception directed and dominated by cities. Infectious bacterial and viral diseases that pass directly from human to human with no intermediate host are therefore the diseases of civilization par excellence: the peculiar hallmark and epidemiological burden of cities and of countryside in contact with cities. They are familiar to almost all contemporary humankind as the ordinary diseases of childhood: measles, mumps, whooping cough, smallpox and the rest.

Most and probably all of the distinctive infectious diseases of civilization transferred to human populations from animal herds. Contacts were closest with the domesticated species, so it is not surprising to find that many of our common infectious diseases have recognizable affinities with one or another disease afflicting domesticated animals. Measles, for example, is probably related to rinderpest and/or canine distemper; smallpox is certainly connected closely with cowpox and with a cluster of other animal infections; influenza is shared by humans and hogs.

Human populations may become diseased by intruding upon one or another disease cycle established among wild animals. Bubonic plague, at home among burrowing rodents, yellow fever at home among monkeys, and rabies at home among bats are examples of the more lethal of such infections.

The size of dose required to infect a new host, the length of time during which the infection may be transferred from one person to another, modes of such transfer, and customs affecting opportunities for exchanging infections, all play roles in determining how many get sick and when. Not infrequently a disease requires a massive, megalopolitan concentration of human hosts to survive indefinitely

Careful statistical study of the way measles propagates itself in modern urban communities shows a wave pattern cresting in periods of time just under two years. Moreover, it has recently been demonstrated that to keep this pattern going, measles requires a population with at least 7,000 susceptible individuals perpetually in its ranks. Given modern birth rates, urban patterns of life and the custom of sending children to school, where measles can spread very rapidly through a class of youngsters meeting the virus for the first time, it turns out that the minimal population needed to keep measles going in a modern city is about half a million. By scattering out across a rural landscape, a smaller population suffices to sustain the chain of measles infection. The critical threshold below which the virus cannot survive falls between 300,000 and 400,000 persons.

By 1900, therefore, for the first time since cities had come into existence almost five thousand years previously, the world’s urban populations became capable of maintaining themselves and even increasing in numbers without depending on in-migration from the countryside.66 This was a fundamental change in age-old demographic relationships. Until the nineteenth century, cities had everywhere been popula- tion sumps, incapable of maintaining themselves without constant replenishment from a healthier countryside. It has been calculated, for example, that during the eighteenth century, when London’s Bills of Mortality permit reasonably accurate accountancy, deaths exceeded births by an average of 6,000 per annum. In the course of the century, London therefore required no less than 600,000 in-migrants for its mere maintenance.

The homogenization of disease distribution

Only genetic mutation of a disease-causing organism, or a new transfer of parasites from some other host to human beings offered the possibility of devastating epidemic when world transport and communications had attained a sufficient intimacy to assure frequent circulation of all established human diseases among the civilized populations of the world.

On the time scale of world history, indeed we should view the “domestication” of epidemic disease that occurred between 1300 and 1700 as a fundamental breakthrough, directly resulting from the two great transportation revolutions of that age—one by land, initiated by the Mongols, and one by sea, initiated by Europeans.

Civilized forms of person-to-person infection had entered the scene with the rise of cities and the development of intercommunicating human herds of half a million or so. Initially this could only occur at selected spots on the globe, where agriculture was especially productive and local transport nets made concentration of resources into urban and imperial centers relatively easy. For millennia thereafter, these civilized infections played a double role. On the one hand, they cut down formerly isolated populations that came into contact with disease-bearers from one or another of the civilized centers, and thereby facilitated the process of “digestion” of small, primitive groups into the body politic of persistently expanding civilized communities. On the other hand, these same diseases enjoyed an imperfect circulation within civilized communities themselves, and could often therefore invade a particular city or rural community with almost the same lethal force they regularly exerted vis-à-vis isolated populations.

After 1300, contacts between the major civilizations of the Old World became closer and closer. Disease exchanges intensified correspondingly, with frequent disastrous but never quite paralyzing consequences. In the sixteenth and seventeenth centuries, when the Amerindian die-off was at its peak, the homogenization of civilized infectious disease throughout the world gradually attained a new level.

Bacteriology was at least as important as technology. The decay of native numbers and the availability of European populations to occupy such vast and varied emptied spaces both derived from the distinctive modern pattern of epidemiology

More frequent contacts across ocean distances tended to homogenize infectious disease. As this took place, sporadic and potentially lethal epidemics gave way to endemic patterns of infection. To be sure, in the first centuries after ships began to ply the oceans of the earth and united all the coastlines of the world into a single intercommunicating network, the process of homogenization of disease distribution involved expansion of some diseases onto new ground.

Such a contrast between radical decay of previously isolated communities on the one hand and a globally enhanced potential for population growth among disease-experienced peoples on the other, acted to tip the world balance sharply in favor of the civilized communities of Eurasia. The cultural and biological variety of humankind was reduced correspondingly, as the age-old process of epidemiological disruption and absorption of survivors into the expanding circle of civilized society accelerated everywhere on earth.

Achievements and unexpected outcomes of the modern medicine on the infectious diseases

Nearly all microparasites are too small to be seen with the unaided human eye, and this meant that until the invention of the microscope and other elaborate aids to human powers of observation, no one was able to understand or do much to control encounters with such organisms. relations with microparasites remained until the nineteenth century largely biological, that is, beyond or beneath human capacity for conscious control. It was not really until after 1850 or so that the practice of medicine and the organization of medical services began to make large-scale differences in human survival rates and population growth.

By the end of the nineteenth century, scientific medicine had discovered effective means to counter the dread disease. Robert Koch discovered the bacillus causing cholera, thereby, giving enormous new impetus to the germ theory of disease. Not only that: methods for guarding against cholera became self-evident as soon as the nature of the infection was known. Chemical disinfectants and heat could kill the bacillus; careful handling of sufferers could guard against passing the disease to others; and by 1893 a vaccine against cholera had been developed.

A number of other infectious diseases of long-standing importance also quickly succumbed to the new techniques bacteriologists had learned to command. Thus typhoid fever was first identified as a distinct disease in 1829; its causative bacillus was discovered and an effective vaccine developed by 1896; and in the first decade of the twentieth century mass inoculations against typhoid proved capable of checking the disease. Diphtheria bacilli were identified in 1883, and an antitoxin was proved effective in 1891. Bacilli in milk were brought under control by pasteurization. Robert Koch won instant fame in 1882 by announcing the discovery of the causative bacillus. Almost fifty years later, in 1921, a partially effective vaccine against tuberculosis was finally produced. It was between 1909 and 1912 that the role of the louse in spreading typhus fever was figured out. By 1937, however, the development of a cheap and effective vaccine deprived yellow fever of its former significance for human life. New chemicals—DDT, sulfas, penicillin, Atabrine, for instance—made formerly formidable diseases easy to prevent or cure

Since the 1940s, therefore, the impact of scientific medicine and public health administration upon conditions of human life has become literally worldwide. In most places epidemic diseases have become unimportant, and many kinds of infection have become rare where they were formerly common and serious.

One interesting and ironic development has been the appearance of new diseases of cleanliness. The chief example of this phenomenon was the rising prevalence of poliomyelitis in the twentieth century, especially among the hygienically most meticulous classes. It seems clear that in many traditional societies minor infection in infancy produced immunity to the polio virus without provoking any very pronounced symptoms; whereas persons whose sanitary regimen kept them from contact with the virus until later in life, often suffered severe paralysis or even death.

Another sort of epidemic disease whose fixture among mankind remains at least potentially significant is well illustrated by the influenza epidemic of 1918–19. Influenza has been around a long time, and is remarkable both for the rapidity of its spread, the brevity of the immunity it confers, and the instability of the virus that causes the disease. Influenza virus is itself unstable and alters details of its chemical structure at frequent intervals. Any new and widespread epidemic is therefore almost sure to originate with a virus that has changed enough to escape the antibodies last year’s vaccine can create in human bloodstreams.

Lesson for the future

Humanity is in course of one of the most massive and extraordinary ecological upheavals the planet has ever known. Not stability but a sequence of sharp alterations and abrupt oscillations in existing balances between microparasitism and macroparasitism can therefore be expected in the near future as in the recent past.

In any effort to understand what lies ahead, as much as what lies behind, the role of infectious disease cannot properly be left out of consideration. Ingenuity, knowledge, and organization alter but cannot cancel humanity’s vulnerability to invasion by parasitic forms of life. Infectious disease which antedated the emergence of humankind will last as long as humanity itself, and will surely remain, as it has been hitherto, one of the fundamental parameters and determinants of human history.

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