2. WHAT ELSE CAN WE LEARN FROM EMERGING INFECTIONS?
Of course, many new infectious diseases do not develop into serious public health problems (McMichael 2001; Patz & Confalonieri 2004). Some do little more than establish a toehold at the margins of human society. Others flicker sporadically. Some, such as the ‘English Sweats’ of the sixteenth century, may circulate for decades and then apparently disappear. In the meantime, we cannot know what the future trajectory of the new disease will be. Twenty years after the initial spread of HIV/AIDS we are aghast at the scale of the pandemic. Meanwhile, we can learn things from all such emerging infections. In particular, they widen our understanding of the ecology of infectious diseases, and of the often distinctive adaptation mechanisms that boost the pathogen’s survival or spread.
The science of emerging infections used to be a lot simpler. In the words of the biblical Old Testament:
The Lord shall smite thee with a consumption, and with
a fever, and with an inflammation, and with an
extreme burning.
(Deuteronomy 28:22)
Indeed, when wrathful, the Lord was apparently partial to quite a bit of smiting. Strange and often fatal diseases were thus dispensed as Divine Retribution.
The Revelation of St John the Divine—part of the Apocrypha that follows the Christian bible’s New Testament, and written in about AD 100—gives a colourful account of this category of disease. St John describes the Four Horsemen of the Apocalypse, the fourth (pestilence, riding on a white horse) being the harbinger of near-certain death.
The four horsemen are instructive in another sense. They are: War, Conquest, Famine and Pestilence. Two millennia ago in the eastern Mediterranean region, these were the four main recurring scourges of human happiness, health and survival. As public health threats, they are all conceptualized in population-level terms. An individual may be starving or malnourished, but it is the population at large that undergoes famine. An individual may contract an infection, but it is pestilence that sweeps through the whole population. That is, these terms are attuned to the notion of population-based phenomena as determinants of health and survival. This is a perspective that is often missing in the modern, popular discussion of the determinants of health (Reiter & Sprenger 1987; McMichael 2002).
Contemporary discussion, therefore, needs to incorporate a clearer understanding of infectious disease within an ecological framework. It cannot be mere chance that there has been an upturn in the tempo of new and spreading infectious disease in recent decades. Therefore, can we clarify, and assess the relative roles of the environmental and social factors in infectious disease emergence?
Both those two adjectives should be treated liberally. ‘Environmental’ refers to the physical circumstances of contact between pathogen and human. Environments can change at the micro, meso and macro scales. ‘Social’ encompasses community-level and individual-level behaviours, contact networks, choices of technology, etc. ‘Social’ also embraces political and cultural circumstances, and the distribution of advantage and disadvantage of varying vulnerability.
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