The spillover of viruses from animal to human populations is an enduring threat to human health. The COVID-19 pandemic is only the most recent instance. Over the many millennia that the human species has existed, there were 219 viruses known to infect humans as of 2012.1 Given this modest total, the rate of novel infections in recent years is striking as major outbreaks of the following diseases have all occurred in recent decades: HIV/AIDS (first detected in the mid-20th Century, worldwide infections since the 1980s), Zika (first detected in 1940, major outbreaks since 2007), Ebola (first detected in the 1940s, major outbreak in 2014), SARS coronavirus (2003), H1N1 influenza (2009), and Middle East Respiratory Syndrome (MERS 2012).2

The threat of zoonotic spillover is accelerating. It is estimated that there are over 1.5 million unknown viruses in animal reservoirs; over 600,000 (perhaps as many as 850,000) of these viruses have the potential to infect humans.3 Sustained collaboration between the natural sciences and the social sciences is essential if we are to anticipate and come to terms with zoonotic spillover: “understanding of the social factors shaping the dynamics of interest, as well as more explicit and effective addressing of policy issues within the research framework are obvious benefits from working with social sciences; for the social sciences, more detailed understanding of the biological processes of interest can both raise vital new questions and beneficially refine research approaches.”4 We offer two social science contributions to the study of zoonotic spillover: normal accidents5 and treadmill theory.6 These concepts help us identify high-risk activities and social structures and processes that heighten the likelihood of zoonotic spillover.

In Normal Accidents, Charles Perrow’s immediate concern is the failure of tightly integrated and complex technological systems – e.g., the Challenger disaster. Although each individual component is engineered to meet exceedingly high standards (e.g., failures occurring 1 time in a million trials), when every component must work, there are thousands of them, and trials are repeated multiple times in a range of conditions, the chances of a catastrophic accident are surprisingly high – in a statistical sense, accidents are normal. Although the normal accidents and zoonotic spillover are not completely analogous, the logic of normal accidents offers lessons for thinking about zoonotic spillover.

Zoonotic spillover is quite rare. Multiple barriers impede “the flow of a pathogen from a reservoir host to a recipient host. Spillover requires the pathogen to pass every barrier and thus can only occur when gaps align in each successive barrier within an appropriate window in space and time. Consequently, zoonotic spillover is a relatively rare event, and although humans are continually exposed to many potentially infectious pathogens that are derived from other species, most of these microorganisms cannot infect or cause disease in humans.”7 Pathogen spillover events can be linked with human-animal interactions with both natural hosts (such as bats) and intermediate hosts (such as domesticated animals). Although the risk posed by a single encounter or even several hundred encounters is quite low, the chance of spillover grows as the number of encounters grows. If encounters are frequent enough, spillover becomes “normal” – in the Anthropocene Epoch these encounters are growing rapidly – and in some cases exponentially.

Treadmill theory – the second social scientific contribution emphasized in this essay8 – sheds light on social processes that make encounters with novel viruses more likely and increase the risk of such encounters. Several human activities have already caused and elevate the risk of future zoonotic spillover, including climate change, hunting, human migration, landscape changes, livestock trade, and environmental contamination -- and these threats have been accelerated with the process of globalization.9 A treadmill spans the biophysical and the social/cultural realms and refers to an anthropocentric process wherein: (1) powerful organizations appropriate and transform nature to amass power and capital, (2) competition among these organizations propels accelerating degradation of the environment, and (3) these organizations obscure, distort and suppress information about the environmental damage. The macrosocial context, the organizations at the center of them, and the elites that command these organizations make these treadmills distinct.10 A “treadmill of production” is derived from and propels economic competition; a “treadmill of destruction” is fueled by geopolitical and military rivalries. Treadmills of production and destruction accelerate environmental degradation and heighten the risk of zoonotic spillover.

The dangers posed by treadmills are distinctive because powerful organizations exploit nature and extract resources in competition with rivals that threaten their organizational vitality – and potentially their survival. These powerful organizations are caught on a treadmill of environmental degradation – and the pace of degradation quickens. For example, unless corporations keep pace in resource extraction and production processes (even if these processes result in dangerous discharges to the environment), they will often be at a severe disadvantage and profits will shrink (or disappear altogether). Treadmills of destruction are driven by arms races and wars – leading to spiraling degradation of the environment. In the context of war and arms races, failing to keep pace with adversaries can lead to debilitating losses and total military defeat. These organizations distort and suppress information about the environmental degradation they are causing.

In recent decades – and continuing (if not accelerating at present) – treadmill dynamics are exacerbating the threat of zoonotic spillover. Two recent episodes of zoonotic spillover – AIDS and Ebola – highlight the risks posed by a “treadmill of destruction.” AIDS and Ebola spillovers are thought to be the result of the consumption of bushmeat and anthropogenic land-use changes. The killing of wild animals for human consumption exposed humans (and their domesticated animals) to viruses harbored by these wild animals.11 The problem is exacerbated by anthropogenic land-use changes that alter human and wildlife migration patterns. Due to this repeated exposure, the AIDS virus and the Ebola virus made its way to the human population. For the past several decades, national armies (albeit poorly supplied), guerrilla armies and a wide range of irregular armed forces have engaged in a prolonged and multi-sided conflict in the Sahel region of Africa. Many of these armed forces have been living off the land – often in kleptocratic fashion. This includes unsustainable consumption of bushmeat – purchased in markets or killed and consumed directly by armed forces or civilians attempting to survive in the context of disrupted livelihoods and living arrangements.12 Leading conservation biologists have documented that the regions impacted by these wars are driving an unprecedented and potentially irreversible wave of extinctions.13 This systematic killing and consumption of wild animals, exacerbated by land-use changes, constitutes a sharp spike in the number of human encounters with novel viruses and elevates the risk of zoonotic spillover.

The dynamics at work in the Amazon rainforest highlight the threats posed by a “treadmill of production.” Whereas the treadmill of destruction is focused on military and geopolitical competition, a treadmill of production is driven by economic competition – growth and profits. With Brazilian governmental support, vast tracts of the Amazon forest are being cleared to make way for large agricultural operations – ranching prominent among them. This aggressive clearcutting – including setting fire to large swaths of the forest -- may push the deforestation to a tipping point that changes regional weather patterns and the global climate.14 The Amazon rainforest is an exceptional ecological hotspot. Hundreds of thousands of species are found in this forest – and only in this forest. It is estimated that the Amazon rainforest is home to 10% of all species on Earth.15 This implies that roughly 10% of all viruses are hosted by these animals. As their unique ecosystem shrinks or disappears altogether, animals will be stressed (many will go extinct) and they (and the tens of thousands of viruses they host) will come into sustained contact with cattle herds and with people.

We endorse calls for interdisciplinary collaborations to anticipate zoonotic spillover16 -- and we believe the social sciences can make valuable contributions. The concept of normal accidents reminds us that even rare events are likely to occur with repeated trials over a range of contexts. Treadmill theory points to human organizations and processes that generate repeated exposure to novel viruses and heighten the risk of these exposures. Treadmills are a uniquely pernicious type of human activity. Zoonotic spillover is exacerbated by environmental transformation and biodiversity degradation propelled by treadmills of destruction and production.

Read the entire Evolutionary Sociology series:

  1. Introduction: Nothing In Sociology Makes Sense Except in the Light of Evolution by Russell Schutt, Rengin Firat, and David Sloan Wilson
  2. Social Science Contributions to the Study of Zoonotic Spillover: Normal Accidents and Treadmill Theory by Michael Ryan Lengefeld
  3. Is Video Chat a Sufficient Proxy for Face-to-Face Interaction? Biosociological Reflections on Life during the COVID-19 Pandemic by Will Kalkhoff, Richard T. Serpe, and Josh Pollock
  4. Natural and Sociocultural Selection: Analyzing the Failure to Respond to the C-19 Pandemic by Jonathan H. Turner
  5. Bringing Neuroscience and Sociology into Dialogue on Emotions to Better Understand Human Behavior by Seth Abrutyn
  6. Speculations About Why Sociological Social Psychology Largely Elides Evolutionary Logic by Steven Hitlin
  7. The Coronavirus Pandemic, Evolutionary Sociology, and Long-Term Economic Growth in the United States by Michael Hammond
  8. Institutionalization of Animal Welfare and the Evolution of Coronavirus(es) by Erin M. Evans
  9. The Coronavirus in Evolutionary Perspective by Alexandra Maryanski
  10. Gene-Culture and Potential Culture-Gene Coevolution: The Future of COVID-19 by Marion Blute
  11. For God’s Sake! What’s All This Fuss About a Virus? by Andrew Atkinson
  12. How Covid-19 Reminds Us We Are More Alike Than Different by Rosemary L. Hopcroft
  13. From the Middle: Sites of Culture, Cooperation, and Trust in Risk Society by Lukas Szrot
  14. Evolution Does Not Explain Tyranny: COVID-19 Could Have Led To Many Fewer Deaths If Tyranny Had Been Less Prevalent in Washington, D.C. by Richard Devine
  15. The Epidemic and the Epistemic: An Exercise in Evolutionary Sociology by Doug Marshall


[1] Woolhouse, Mark, Fiona Scott, Zoe Hudson, Richard Howey and Margo Chase-Topping. 2012. “Human Viruses: Discovery and Emergence.” Philosophical Transactions of the Royal Society B 367:2864–2871. doi:10.1098/rstb.2011.0354.

[2] World Health Organization. 2020. World Health Topics. Web-site:, accessed May 1, 2020.

[3] Carroll, Dennis, Peter Daszak, Nathan D. Wolfe, George F. Gao, Carlos M. Morel, Subhash Morzaria, Ariel Pablos-Méndez, Oyewale Tomori, Jonna A. K. Mazet. 2018. “The Global Virome Project: Expanded Viral Discovery Can Improve Mitigation.” Science 359:872-874. DOI: 10.1126/science.aap7463.

[4] J.L.N. Wood, M. Leach, L. Waldman, H. MacGregor, A.R. Fooks, K.E. Jones, O. Restif, D. Dechmann, D.T.S. Hayman, K.S. Baker, A.J. Peel, A.O. Kamins, J. Fahr, Y. Ntiamoa-Baidu, R. Suu-Ire, R.F. Breiman, J.H. Epstein, H.E. Field, A.A. Cunningham. 2012. “A Framework for the Study of Zoonotic Disease Emergence and its Drivers: Spillover of Bat Pathogens as a Case Study.” Philosophical Transactions of the Royal Society B: Biological Sciences. 367:2881-2892.

[5] Perrow, Charles. 1984. Normal Accidents: Living with High-Risk Technologies New York: Basic Books.

[6] Allan Schnaiberg. 1980. The Environment: From Surplus to Scarcity. New York: Oxford University Press; Gregory Hooks and Chad L. Smith. 2004. "The Treadmill of Destruction: National Sacrifice Areas and Native Americans.” American Sociological Review 69:558-76.

[7] Plowright Raina, Colin Parrish, Hamish McCallum, Peter Hudson, Albert Ko, Andrea Graham and O. Lloyd Smith 2017. “Pathways to Zoonotic Spillover. Nature Reviews: Microbiology 15:502-10.

[8] Although not examined here, there are other social scientific insights that can help us understand zoonotic spillover. A well-known framing of the human impacts on the environment centers on the rapid growth in the number of humans (e.g., Paul Ehrlich. 1968. The Population Bomb Ehrlich. Rivercity Press). This insight is refined by considerations of consumerism, affluence and technology (see, for example, Donella Meadows. The Limits to Growth: A Report for the Club of Rome's Project on the Predicament of Mankind. New York: Universe Books).

[9] Thompson, R.C. Andrew. 2013. “Parasite Zoonoses and Wildlife: One Health, Spillover and Human Activity.” International Journal for Parasitology 43:1079-1088. See also: Aguirre, Alonso. 2017. “Changing Patterns of Emerging Zoonotic Diseases in Wildlife, Domestic Animals, and Humans Linked to Biodiversity Loss and Globalization.” ILAR Journal 58:315–318 ; Carroll et. al., op. cit.; Rodriguez-Morales et al. 2020. “History is repeating itself: Probable zoonotic spillover as the cause of the 2019 novel Coronavirus Epidemic.” Le Infezioni in Medicina, n. 1, 3-5.

[10] Gregory Hooks, Michael Lengefeld, and Chad L. Smith. Forthcoming. “Recasting Treadmills of Production and Destruction: New Theoretical Directions.” Sociology of Development. On the treadmill of production, see: Allan Schnaiberg. 1980. The Environment: From Surplus to Scarcity. New York: Oxford University Press; Allan Schnaiberg, Alan and Kenneth A. Gould. 1994. Environment and Society: The Enduring Conflict. New York: St. Martin’s Press. Regarding the treadmill of destruction, see: Gregory Hooks and Chad L. Smith. 2004. "The Treadmill of Destruction: National Sacrifice Areas and Native Americans.” American Sociological Review 69:558-76; Gregory Hooks and Chad L. Smith. 2005. “Treadmills of Production and Destruction: Threats to the Environment Posed by Militarism.” Organization & Environment 18:19-37.

[11] Wolfe, Nathan, Peter Daszak, A. Kilpatrick, and Donald Burke. 2005. “Bushmeat hunting, deforestation, and prediction of zoonoses emergence.” Emerging Infectious Diseases, 11(12):1822‐1827.

[12] McPake, Barbara, Sophie Witter, Sarah Ssali, Haja Wurie, Justine Namakula, and Freddie Ssengooba. 2015. “Ebola in the context of conflict affected states and health systems: case studies of Northern Uganda and Sierra Leone.” Conflict and health, 9, 23.

[13] Jose Carlos Brito, Sarah M. Durant, Nathalie Pettorelli, John Newby, Susan Canney, Walid Algadafi, Thomas Rabeil, Pierre-Andre Crochet, Juan Manuel Pleguezuelos, Tim Wacher, Koen de Smet, Duarte Vasconcelos Goncalves, Maria Joana Ferreira da Silva, Fernando Martinez Freiria, Teresa Abaigar, Joao Carlos Campos, Pierre Comizzoli, Soumia Fahd, Amina Fellous, Hamissou Halilou Malam Garba, Dieng Hamidou, Abdoulaye Harouna, Mahamat Hassan Hatcha, Abdullah Nagy, Teresa Luisa Silva, Andack Saad Sow, Candida Gomes Vale, Zbyszek Boratyński, Hugo Rebelo and Silvia B. Carvalho. 2018. “Armed Conflicts and Wildlife Decline: Challenges and Recommendations for Effective Conservation Policy in the Sahara-Sahel.” Conservation Letters. 2018;11:e12446. DOI: 10.1111/conl.12446

[14] Piotrowski, Matt. 2019. Nearing the Tipping Point: Drivers of Deforestation in the Amazon Region. Washington, DC: Inter-American Dialogue.

[15] Vicky Stein. 2019. “9 Numbers You Need to Know to Understand the Amazon Fires.” PBS News Hour (August 26), available online at:, accessed June 1, 2020.

[16] Plowright et al. 2017, op cit.; Wood et al. 2012, op cit.