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Historical transitions, past and present
- Environmental and social influences on emerging infectious diseases: past, present and future


During many millennia of human cultural evolution, dispersal around the world, and subsequent interpopulation contact and conflict, there have been several distinct transitions in human ecology and in interpopulation interactions that have profoundly changed the patterns of infectious disease in human populations. The main transitions have been as follows.

(i) Prehistoric transition.

(ii) Historic transitions:

(1) first (local), 5–10 000 years ago;


(2) second (continental), ca.1–3000 years ago;
(3) third (intercontinental), from ca. AD 1500;
(4) today (global), fourth historical transition.

The prehistoric transition began, several million years ago, with the move from tree dwelling to savannah. This entailed changes in exposures to mosquito and tick species. Likewise, the growing reliance of early Homo species on meat eating, from ca. 2 Myr ago, and associated activities such as the use of animal skins and fur would have increased exposure to enzootic agents and their vectors (including lice). The subsequent radiation of these upright-walking hunter-gatherers into unfamiliar environments would have exposed them to various new parasites.

This early transition also illustrates the close interrelationship between the behavioural, social and environmental domains, as interacting influences on the emergence of infectious disease. Under evolutionary pressures, human behaviours slowly changed. The behavioural move to ground-dwelling upright-walking existence entailed various environmental changes. There were also consequent changes in social relationships, in family and tribal groupings, and in patterns of day-to-day interaction between these hunter-gatherer hominids. Clearly, it is likely to be misleading to attempt to make a clear-cut differentiation of environmental and social influences on emerging infectious diseases.

Following the early emergence of agriculture and livestock herding, which began ca. 10 000 years ago in the eastern Mediterranean, three great transitions in human– microbe relationships, occurring on an increasingly large scale, are readily recognizable. These have been well described by the historian William McNeill (McNeill 1976). These are referred to as ‘historic’ transitions in that they occurred after the advent of early writing, when recorded ‘history’ became possible.

(a) First historic transition

Early human settlements, from ca. 5000–10 000 years ago, enabled enzootic pathogens to enter Homo sapiens (Weiss 2001). Many mutant microbes, originating from husbanded animals and ‘urban’ pest species (rodents, flies, etc.) would, by chance, have made contact with members of the unfamiliar human species. Most such microbial contacts must have failed. However, some, just as with HIV/AIDS, the Nipah virus and SARS in recent times, would have survived and prospered. They were the progenitors of today’s textbook infections: influenza, tuberculosis, leprosy, cholera, typhoid, smallpox, chicken pox, measles, malaria, schistosomiasis, and many others.

(b) Second historic transition

Early Eurasian civilizations, now large and powerful, came into military and commercial contact, ca. 1500– 3000 years ago, swapping their dominant infections. Rome, China and the eastern Mediterranean swapped their germ pools, often with disastrous results—such as the Justinian Plague of AD 542 that devastated Constantinople and the Roman Empire. The historical record shows that China suffered a series of massive epidemics during these times (McNeill 1976).

Following this second great historical transition, a trans-Eurasian equilibration of infectious disease agents, European populations were becoming genetically and culturally attuned to many of these now endemic or recurring infectious diseases. Presumably, the same was true at the other end of Eurasia, in the ancient, vast civilization of China. Contact with the bubonic plague in Europe, however, had only been occasional, and the disaster of the Black Death in the mid-fourteenth century came at the end of this second transitional period.

(c) Third historic transition

The third transition resulted from European exploration and imperialism, beginning ca. AD 1500 and continuing over much of the past five centuries. This caused the trans-oceanic spread of often-lethal infectious diseases. The devastating impact of the repertoire of infections taken to the Americas by the Spanish conquistadores is well known. Similar processes occurred with European explorations of the Asia-Pacific region, with European settlement in Australia, and with the trans-Atlantic slave trade.

An interesting sidelight on this third transition comes from Charles Darwin’s two-month visit to the east coast of Australia in 1836, en route home after his famous stopover in the Galapagos islands. In his journal Darwin (1839) writes, following his observations of diseases in Australian Aboriginals:

Besides these several evident causes of destruction, there appears to be some more mysterious agency generally at work. Wherever the European has trod, death seems to pursue the aboriginal. We may look to the wide extent of the Americas, Polynesia, the Cape of Good Hope, and Australia, and we shall find the same result.

Darwin later continues:

It is certainly a fact, which cannot be controverted, that most of the diseases that have raged in the islands during my residence there, have been introduced by ships; and what renders this fact remarkable is that there might be no appearance of the disease among the crew of the ship which conveyed this destructive importation.

Here, half a century before the elucidation of the germ theory, this great naturalist was once again making incisive observation and inference.

(d ) Fourth historic transition

Today, we are living through the fourth great historical transition. This time the scale is global and changes are occurring on many fronts. The spread and increased lability of various infectious diseases, new and old, reflect the impacts of demographic, environmental, social, technological and other rapid changes in human ecology. Global climate change, one of the greatest of the human-induced global environmental changes now underway, will have diverse impacts upon the patterns of infectious disease occurrence.

The globalization of our economic activities and culture, the rapidity of distant contact, the spread and intensification of urbanization, and our increasing reliance on either intricate or massive technology, are reshaping the relations between humans and microbes. In particular, we are destabilizing ecosystems in ways that favour the proliferation of the r species—that is, those small opportunistic species that (in contrast to the larger K species such as ourselves) reproduce rapidly, invest in prodigious output rather than intensive parenting, and have mechanisms to efficiently disperse their offspring. Pathogens are typical r species, and they live today in a world of increasing opportunity.

Figure 1 makes a crude attempt to compare the relative importance of major categories of environmental and social factors to each of the four historical transitions. Note, however, that the relativities are more meaningful within each of the four transitions, rather than between the transitions. This is a rather inexact exercise, but it underscores how configurations of social and environmental influences change. It also shows the more intensive set of influences that apply in today’s world.

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