Pestilence and environmental upheaval are ancient bedfellows. It is likely that waves of the European black death were linked to Asiatic climate change. Fifteen years or so after successive climatic fluctuations rendered the Karakoram mountains inhospitable to rodents, outbreaks of plague hit European harbours. With rodent numbers decimated, fleas carrying Yersinia pestis sought other hosts, including human populations, the disease then sauntering the slow medieval trade routes to Europe.

Although covid-19 seemed to fall from a clear sky, there is evidence linking it—and other recent zoonotic disease outbreaks including MERS-CoV, Zika and Ebola—to anthropogenic environmental change. The global decimation of habitat forces birds and animals ever closer to human populations. Deforestation has displaced bat populations—which may be linked to covid-19—into gardens, increasing the likelihood that viruses will shift to human hosts. Global temperature change pushes natural populations from their traditional environments, taking their viruses with them. The disruption of balanced ecosystems undoes natural constraints on pathogens: the explosion of Lyme disease in the US is associated with the loss of habitat for opossums and chipmunks—both highly effective at keeping tick populations in check.

Modern farming, particularly the pressure to produce inexpensive meat at scale, is a powerful driver of zoonotic spillover—viruses jumping the species barrier. Although spillover is probably as old as humankind—and was intensified with the shift to agriculture some 12,000 years ago—modern animal rearing is particularly adept at facilitating it. Among the major sources are overcrowding of farmed livestock, appalling rearing conditions, and widescale use and misuse of antibiotics. Intensive poultry production in China and elsewhere has been convincingly linked to the emergence of dangerous avian influenzas. A 2015 US study showed that Europe and the US are the world’s leading exporters of swine flu. Other critical sources of virus spillover are smallholders—many forced into the traffic and cultivation of wild animals by competition from large-scale agribusiness. 

As covid-19 demonstrates, increases in human mobility and population density further facilitate the spread of zoonotic viruses, and magnify the challenges of response. Poverty compounds the problem: overcrowding, multi-occupancy, poor ventilation, difficulty social distancing, and unsanitary conditions combined with generally worse underlying health speed up transmission and exacerbate health burdens. 

The rapid development of vaccines for covid-19 has been a triumph, both of biotech and regulation—delivering a safe and effective vaccine to market in the twinkling of a regulatory eye was breath-taking. It showcased our technological sophistication, our ability deftly to respond to serious human threats. But there is a danger in relying exclusively on biomedical responses. They risk shrinking our vision to the micro-dot of a pathogen, tempting us to extract the disease from the conditions that enable it to spillover and flourish. In doing so we menace our ability to understand the critical links between human health and the complex natural and man-made systems on which it depends. If covid-19 has taught us anything it is that we are links in long chains of wellbeing. Until we change our relationship to the global ecosystems on which we depend, we will lurch from crisis to crisis.

Among the impediments routinely trotted out to environmental change are financial—that such a global transformation is improbably, unachievably expensive. But The Economist has recently put the global economic cost of covid-19 in GDP foregone at $10 trillion for 2021-21, surely a figure big enough to focus the minds of a few world leaders.

There will be many lessons learnt from covid-19. Few aspects of our lives will be unaffected. But now is the time to think long and hard about the links between human health and the integrity of our global environments. One health—along with its stablemates eco-health and planetary health—is a trans-disciplinary movement designed to promote optimal human health through acknowledging the strength of our connection to animals, plants, and their shared ecosystems. It places our dependence on our environments and their biota front and centre. And as covid-19 has made transparent, we have never needed its perspectives so urgently. 

Julian Sheather is ethics manager, BMA. The views he expresses in his opinion pieces are entirely his own.

Competing interests: none declared.

Editor’s note: This article was corrected on 29 April 2021 to change the spelling of Lyme Disease.