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Every disease that has caused epidemics and changed the philosophical traditions of societies throughout human history has originated in nonhuman animals and “jumped the species barrier” into humans. There is no meaningful separation between animal and human diseases when discussing the impact of disease on human history.
It is important to emphasize that because humans are mammals, diseases found in other nonhuman animals, especially other mammals, often cross readily into humans. The most important diseases are infectious and highly contagious. Noncontagious diseases have had little or no impact on history. By definition, infectious diseases are capable of spreading rapidly from infected to healthy individuals. Infected individuals either die or recover fully within a short period of time; those individuals who recover typically acquire immunity against further infection by the same illness.
Numerically, the single greatest documented epidemic in human history was an influenza outbreak that killed 40 million people at the end of World War I. The epidemic with the greatest recorded impact was the bubonic plague that killed over 25 percent of the people in Western Europe in the mid-fourteenth century. Despite a lack of documentation, however, the epidemics with the greatest overall impact both on human populations and history were the series of outbreaks that spread through the Americas shortly after contact with Europeans and their domestic animals. These epidemics spread through populations previously unexposed to the diseases of Eurasia, typically causing 90–95 percent mortality, especially within communities suffering from multiple traumas associated with colonialism—violence, slavery, subsistence collapse. Overall these diseases may have killed as many as 100 million people in the Americas.
Prominent examples of infectious diseases that have crossed from other animals into humans include smallpox, cholera, tuberculosis, bubonic plague, and influenza. Although AIDS represents a major potential health problem in the modern world, it is contagious, but neither infectious nor acute. In recent years there have been panics over other animal diseases such as hoof-and-mouth disease, Hanta virus, and so-called mad-cow disease, which may not be a disease in the usual sense at all. These pathological conditions are trivial compared to the impact of the other diseases listed, yet they have received more publicity, perhaps because of ignorance and media-inspired fear, combined with the fact that most people do not understand how various diseases are transmitted.
Most infectious animal diseases that jump to humans are caused by bacteria and viruses whose small size renders them highly volatile and transmissible as aerosols, hence more likely to be transmitted from one individual to another, which is the basis of contagion. A few diseases, such as malaria and sleeping sickness, are caused by protistans, single-celled eukaryotic organisms that are much larger than bacteria or viruses. The larger size of protistans means they cannot be transmitted as aerosols, and hence are transmitted primarily by injection, for example, through insect bites, rendering them much less infectious.
Most infectious disease organisms co-evolve in interactions with other nonhuman species. These nonhuman species have evolved an immune response to the disease-causing organisms, so they are not serious threats either to health or population numbers in their original host species. What renders most of infectious diseases so virulent in human populations is that when first exposed, humans have no evolved immune response to these pathogens; for example, smallpox is related to bovine pox, which causes minor problems in cattle but is often fatal in its mutated form in humans. Similarly, the AIDS virus is closely related to a viral infection that occurs in African primates, where it only causes mild influenza-like symptoms. Other examples include measles, which is closely related to the ungulate disease rinderpest; tuberculosis, which is closely related to a similar disease in cattle; and influenza, which is actually a complex of viral diseases derived repeatedly from similar pathogens occurring in pigs (swine flu) and birds such as ducks and chickens. Recently, malaria was added to this list when it was discovered that the human malarial parasite is closely related to a less virulent form in chimpanzees.
Contagious diseases that manage to cross the species barrier from nonhumans into humans have been a major factor shaping the history of Europe and Asia. A major difference between Europe and Asia as contrasted with the Americas and Africa is that Eurasian cultures domesticated and lived in close association with the animal species that served as the original hosts of these diseases. Domestication of ungulates, especially cattle and swine, set up scenarios whereby humans living on intimate terms with these animals were continually exposed to a wide range of epidemic diseases, which already afflicted the ungulate populations as minor problems. These diseases thrived particularly well in the high densities at which human societies kept cattle and pigs. Farmers are sedentary, living among their own sewage and that of the domestic animals with whom they live in an intimate and symbiotic fashion. In many agrarian societies farmers traditionally took cattle and pigs into their homes at night, both for warmth and to protect their livestock from predators. These conditions both prolong exposure and increase the likelihood of transmission of bacterial and viral pathogens.
Agriculture sustains much higher human densities than the hunting-gathering lifestyles that agriculture replaced. The large concentrations of humans resulting from increased urbanization provided fertile ground for the rapid spread of infectious diseases that originated in other species. Only within the last century did European cities achieve self-sustaining populations, because so many city dwellers died from disease that constant immigration from rural areas was required to sustain urban areas.
The Black Death
Development of world trade routes rapidly increased the dispersal rate of epidemic diseases. By Roman times the populations of Europe, Asia and North Africa had become a giant breeding ground for disease organisms that originated in domestic livestock. Smallpox reached Rome in the second century CE, killing millions of Roman citizens as the Plague of Antoninus. The animal-borne disease with the most profound impact on the course of history in Europe and Asia was bubonic plague. Spread by fleas that pick up the plague bacillus from the fur-bearing mammals that are their normal hosts, plague first appeared in Europe as the Plague of Justinian in 542–543 CE. The most devastating impact of the plague, however, occurred in fourteenth-century continental Europe where it killed as many as 25 million people and became known as the Black Death. In the British Isles alone, plague killed nearly 1.5 million people (25–40 percent of the total population). The main vector for the major outbreak of plague appears to have been furs brought from low-population density areas in central Asia with the opening of trade routes to China in the mid-fourteenth century.
One important, often unappreciated, consequence of the fourteenth-century plague was its profound impact on European philosophy and science. The prevailing European worldview prior to the mid-fourteenth century was mythic and symbolic, rooted in an idea of cyclical time, placing far more emphasis on links between human and nonhuman aspects of the world than did the worldviews that arose after the Black Death.
When plague arrived and began to have devastating impact on local populations, the knowledge base and techniques of this older philosophical tradition were pressed into service, including prayer, medicine based on sympathetic magic, and scapegoating (for example, witch burning). None of these methods proved effective, and the inability to deal with the resulting death and devastation created both widespread panic and subsequent culture-wide depression. The impact of massive, inexplicable loss of life on a society cannot be overestimated. Belief in spiritual traditions and ways of understanding how the world works are crushed, leading to a sense of spiritual desolation.
The experience of the plague, described by some historians as the “greatest biological-environmental event in history” and the “equivalent of nuclear holocaust” by others, forced Western Europe to develop a new way of organizing its perception of reality. Within Christianity the plague led to loss of faith in a benevolent, heedful Creator, leading to persecution and scapegoating of “heretics,” eventually leading to the beginnings of Protestantism and its images of a vengeful, wrathful God.
From a more scholarly perspective, response to the plague experience led to development of an intellectual tradition that separated mind from body, objective from subjective, and human from nature. This led to the beginnings of the Renaissance and development of the Western European “rationalist” scientific tradition, ultimately generating Cartesian dualism, the machine model/metaphor as a way of understanding nonhuman life, and the Baconian- Newtonian worldview. Thus the philosophical and spiritual impact of plague led directly to the “modern” rationalist approach in which experimentation and measurement substituted for observation and experience.
This new way of dealing with reality had numerous positive effects. For example, it led to increased sanitation, which reduced background levels of many contagious diseases. This division of reality into separate spheres of mind and matter provided a powerful methodology for the study and understanding of the “outside” world. It was largely inadequate, however, for understanding inner experience, the human mind, and our relationship with the world of our fellow life forms. Thus, although this dualistic view led to improved sanitation, there was no increased understanding of the natural cycle of disease or the evolution of immune responses.
Old and New Worlds
The importance of animal diseases in shaping both human history and cultural attitudes toward the environment can be illustrated by comparing the Old World (Eurasia and North Africa) with the New World (North and South America). Many cultures in the Americas developed agriculture, but New World agriculture was based almost exclusively around agronomy, for example, corn, potatoes, squash, and beans, rather than on pastoralism, the herding and domestication of ungulates. The only domestic animals in the Americas were dogs, guinea pigs, guanacos (llama and alpaca), and turkeys. Unlike the domesticated ungulates of the Old World, these New World domesticates were never maintained at high densities, humans did not drink their milk, nor were any of these animals except dogs kept in close proximity to humans, as were livestock in the Old World.
Many New World cultures existed at densities comparable to those found in Europe. The Aztec capital of Tenochtitlan may have been one of the largest cities in the world during its heyday, and there is evidence that in central Mexico human populations surpassed the long-term carrying capacity of the land. Similarly, many other New World communities such as cities of the Mayans, Incas, and the Mound Builder cultures along the Mississippi and Ohio River valleys, lived at densities comparable to those found in European and Asian cultures. Despite high population densities, however, epidemic (crowd) diseases appear to be virtually nonexistent in these indigenous New World cultures; this is almost certainly attributable to the absence of domestic ungulates which have been the source of most epidemic diseases (other than bubonic plague) in Europe, Asia, and North Africa. Despite the apparent absence of epidemic diseases in the New World, such diseases may have played a role in the disappearance of some of the larger city complexes in the Americas, possibly because of poor sanitation.
New World Animal Diseases
One of the greatest ironies of the history of animal diseases is that the absence of nonhuman-derived contagious diseases and associated immune responses in New World humans was almost certainly the major factor in the successful invasion of the New World by Europeans and their worldview, which had been dramatically reshaped by their own experience with contagious disease only a few centuries earlier. Europeans sometimes occupied large parts of Africa and Asia, but without the decimating impact of introduced contagious diseases, they did not significantly reduce the indigenous human populations of these areas. As a consequence, as the age of colonialism draws to a close, the indigenous peoples of Africa and Asia have been able to regain social and political control of their own lands because they have remained numerically dominant in their homelands.
In contrast, the introduction of animal diseases into susceptible human populations in the Americas was much more devastating to indigenous human populations than it was during the plague in Europe. It is estimated that 90–95 percent of the indigenous human population of the Americas perished from introduced diseases.
Contrary to popular mythology this holocaust— referred to as the first, or microbial, phase of the European conquest of the Americas—did not begin with the “discovery of the Americas” by Columbus in 1492; it was initiated some time earlier by Basque whalers, Viking settlers, and English fishermen who began landing along the Atlantic coast of the Americas hundreds of years before Columbus arrived in the Caribbean and before other Spanish explorers (conquistadors) arrived in the New World. There is evidence that some tribes originally living along the Atlantic Ocean retreated inland in an effort to escape epidemics that devastated their populations well before the arrival of Cristobal Colon at the end of the fifteenth century.
Despite the success of supposed conquistadors like Cortez and Pizarro, it was smallpox that really led to the collapse of the Aztec and Inca empires. Cortez’s initial 1519 foray into the Aztec civilization was much less successful than his subsequent 1520 effort after smallpox arrived in Tenochtitlan. By the early seventeenth century the indigenous population of Mexico had experienced devastation exceeding 90 percent, falling from an estimated 20 million to less than 2 million. The impact of the disease was demoralizing and crushed the ability of the Aztecs to resist Cortez. Similarly smallpox arrived in Inca territory in 1526, setting up the opportunity for Pizarro’s successful “invasion” in 1531.
There is recorded evidence that 90 percent or more of the indigenous populations were wiped out by these new contagious diseases that arrived with both Europeans and their symbiotic nonhumans. In one well-documented example, the Mandans, one of the most elaborate of the Great Plains cultures, more than 95 percent of their population died after the arrival of smallpox via a Missouri riverboat in 1837. Even given these impacts, however, it is likely that New World populations would have rebounded had their land not remained permanently occupied after European invasions and the subsequent continued colonialism that resulted.
The introduction of alien diseases had a devastating impact on the indigenous peoples of the Americas. If the deaths of 20 to 40 percent of local populations in Europe as a result of plague caused restructuring and rethinking of the role of humans in the world, it is difficult to imagine the spiritual, social, and philosophical impact of the loss of 90 to 95 percent of a population, as occurred in many indigenous peoples of the Americas.
Disease is a major factor in limiting rates of population growth; in fact populations free of the impact of diseases typically outstrip those subject to disease. Indigenous Americans appeared relatively free of epidemic disease prior to the arrival of Europeans. As a consequence indigenous populations had not evolved any immunity to contagious diseases. They did not lack the ability to produce immune responses; instead, the devastation appeared to result from the way in which indigenous populations were exposed to contagion. The major killers of indigenous Americans—smallpox and influenza—were lethal to persons primarily from fifteen to forty years of age, hence the most valuable and productive members of a population, both culturally and demographically. These diseases typically arrived in clusters, punctuated by brief interludes of respite. Thus communities might be ravaged by a series of three or four diseases, followed by a period of remission; communities might subsequently be hit by another bout of a new disease or set of diseases. This combination of periodicity of events with the plurality of the diseases reduced the ability to evolve immune responses.
This pattern generated extreme psychological and spiritual stress. Unable to prevent disease or care for themselves or loved ones, abandoned by kin and other tribal members fleeing the epidemic (and in the process often carrying it to other peoples and communities), many individuals and groups simply gave up hope. Many engaged in activities that only hastened their deaths, such as sweats followed by immersion in cold water. The inability of their traditional holistic methods to treat diseases and control these contagions caused them to lose faith in their healers and medicine people and also to abandon traditional spiritual practices and ceremonies. Because the European invaders had developed some immunity to these diseases, many indigenous peoples assumed that European spiritual and philosophical traditions were superior to their own, which in many cases led to acceptance and adoption of Christianity and its tenets.
The apparent failure of indigenous spiritual traditions, combined with the introduction of new goods and materials, led indigenous peoples to abandon centuries-old traditions of dealing with the natural world, which were based on respect, connection, and conservation. Some peoples may even have blamed the wildlife and the natural world for the epidemics, because it appears that many indigenous peoples associated disease with wildlife and developed cultural traditions that were assumed to minimize the likelihood and impact of disease. For example, the Cherokee assumed that disrespectful treatment of killed deer could lead to crippling illness, such as Lyme disease, which produces crippling effects. The Cherokee blamed the appearance of new diseases on an imbalance in the cosmos caused by failure on their part to perform their rituals correctly. Similarly the Anishinaabe (Chippewa, Ojibway) peoples apparently developed the Mediwiwin healing society and related ceremonies in response to diseases they associated with wildlife, but the diseases were more likely the results of pre-Columbian contact with Europeans.
Affect on Nonhuman Species
Not only humans suffered as a result of the introduction of these diseases. Indigenous peoples depended on many natural populations of animals as sources of food and clothing, including deer, caribou, moose, bison, and beaver, that also experienced massive dieoffs from west of Hudson Bay to the Rocky Mountains during the latter part of the eighteenth century. These deaths probably resulted from disease introduced by Europeans through their domestic animals. It is worth noting that these die-offs were primarily among ungulate populations, which would have been most susceptible to the ungulate-borne contagious diseases characteristic of Europe and Asia. New World carnivores, such as wolves and bears, appeared relatively unaffected by these illnesses, but they suffered as a result of the loss of their ungulate food supplies.
In addition to the impact of disease, additional damage was inflicted upon natural populations of animals when indigenous people destroyed animal populations because of an apparent antipathy towards animals, whom were assumed to have broken their covenants with humans by infecting them with disease. Thus one ironic consequence of the introduction of nonhuman-derived diseases was the destruction of cultural traditions based on respect for nonhumans. Most, if not all, indigenous cultures of North America had philosophical traditions where nonhumans were regarded as creator spirits, and the concept of relatedness was based upon ecological relationships. It has been argued that the devastating impact of introduced disease on these cultures caused them to turn on their nonhuman relatives, allowing some tribes to wipe out local populations of beaver, deer, bison, and wolves, in order to trade furs for European goods and metal.
European Tradition and the Natural World
The invading European tradition, derived primarily from English and Scots cultures, had a very different relationship with the natural world, especially as a result of the Renaissance and the rationalist tradition, which worked to separate itself from any association with the natural world, except as a resource for exploitation. Protestant Christian sects that appeared in Western Europe towards the end of the Renaissance (during the Reformation) developed philosophical traditions that offered no encouragement for investigation into the ways of God’s creatures. God had given humans “dominion” over nonhumans, providing sufficient justification for any action regarding the natural world.
Europeans regarded mountainous country as unpleasant and dangerous, and forests were considered to be even worse. That these places were wild, hence untamed, was sufficient to trigger terror and hostility in western Europeans. The wild (natural world) was so unreasonably fearsome that encroachment of wild creatures into the human domain was highly alarming. A bee flying into a cottage or a bird rapping at the window was enough to frighten people. The English House of Commons rejected a bill in 1604 because a jackdaw flew through the chamber during the speech of its sponsor.
This difference in response to the nonhuman (natural) world continues to manifest itself today in contemporary responses to animal-borne disease. These responses are often extreme in comparison to the actual threat. The most egregious response in recent years has been the slaughter of hundreds of thousands of farm animals, particularly in the British Isles, in response to minor outbreaks of hoof-and-mouth disease and the sporadic and highly unusual occurrence of so-called mad-cow disease.
In the case of hoof-and-mouth disease, the threat is almost exclusively economic. There is little evidence that hoof-and-mouth disease represents any serious threat to human health. Still the economic threat is deemed sufficient enough to destroy hundreds of thousands of animals, mostly because the possibility exists that they may have been exposed to the disease. Can any moral being imagine such a draconian solution if the animals exposed to a potential contagion were Homo sapiens, rather than ungulates? Similarly, wild bison that stray beyond the borders of America’s Yellowstone National Park are summarily slaughtered by agents of the state of Montana on the grounds that these animals might act as reservoirs for the cattle disease brucellosis. The irony in this case is that brucellosis is a disease that evolved in Old World bovids and was introduced into America along with cattle. No bison has ever shown the symptoms of brucellosis, yet the fact that a low percentage of bison test positive for exposure to the pathogen is deemed sufficient reason to kill them.
The response to mad cow disease, more properly called bovine spongiform encephalopathy (BSE), is even more absurd. BSE appears to be one of a group of related pathological conditions that may be caused by prions that appear to be protein molecules capable of self-replication. Other diseases in this category are scrapie in sheep and both kuru and Creutzfeldt-Jakob disease in humans. Such pathological conditions impact the central nervous system (CNS) and gradually destroy the brain. The damage to the CNS is what produces the symptoms that have disrespectfully led to this condition being designated with the term mad cow. A far better and more accurate term would be acutely distressed cow. These apparently prion-based conditions are not directly communicable and can only be passed through consumption of CNS tissue including brain and spinal cord. The only reason these conditions appeared to spread in the United States and England is because slaughterhouses in those countries use “wastes” remaining after butchering to be ground up and added to cattle feed as a protein supplement.
In humans it is obvious that only through consuming CNS material can humans become infected. Outbreaks of kuru in New Guinea are clearly related to the cultural tradition of consuming the brains of other humans as part of a cannibalistic tradition. In England, BSE-type syndromes have shown up in humans who consumed low-grade commercial hamburgers. It seems obvious that banning the use of the waste products of slaughterhouses in both hamburger for human consumption and in cattle feed could stop any possible outbreak, yet commercial pressures have slowed or prevented such moves. Still, the total number of BSE human victims numbers less than twenty, and there is little likelihood of an outbreak or of a human contracting BSE through eating regular beef in the form of roasts or steaks.
Hanta virus is a rodent-borne viral pathogen. There is actually an entire class of Hanta-like viruses in a variety of murid rodents. The one described as Hanta virus appears to have only one species—deer mice, or Peromyscus maniculatus—as its primary host; it does not appear to cause significant health problems in deer mice. In humans, however, this virus causes pneumonia-like symptoms that result in death about 50 percent of the time. This disease is well-known to indigenous peoples of the American Southwest and may be one of the reasons that traditional Dine (Navajo) people destroy a hogan after a person has died in it. In recent years this disease has caused a minor panic in the United States because deer mice are a widespread, common rodent. Hanta does not appear to be transmissible among humans, so it is unlikely to ever become a true epidemic. The number of recorded cases in the United States is less than two hundred since the Center for Disease Control (CDC) has been keeping records.
The major environmental and health-related problems in humans result primarily from close association with domestic animals. This continued proximity has allowed several diseases to jump from their ungulate or avian hosts and cross the species barrier into humans.
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