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Spillover in the Modern World

Human society traded one brand of disease outbreaks for another. In the past, we lacked hygiene, medical infrastructure, and knowledge of what causes disease. Today, we’re better in all three categories, especially in developed countries.

But we’re disturbing ecology more and more, causing more spillover events. We handle familiar spillover diseases like rabies and plague. Yet we struggle to prevent and respond to emerging diseases.

Dense cities, habitat destruction, and concentrated animal farming all contribute to spillover. They force animals to live closer together. Damaged habitat also simplifies ecological relationships and amplifies their effects.

The traffic of viruses goes from the slow exchange of a county road to that of an interstate highway. Diseases can only travel as far as a host can travel. In that way, the ancient world of foot traffic was safe compared to modern same-day international flights.

“We invade tropical forests and other wild landscapes, which harbor so many species of animals and plants—and within those creatures, so many unknown viruses.”

David Quammen, author of Spillover: Animal Infections and the Next Pandemic, wrote in the New York Times

Southern Asia holds the most opportunities for spillover:

  • Dense human population
  • Mobility between cities
  • Lack of hygiene
  • Unregulated wet or bushmeat markets
  • Recent deforestation
  • Concentrated animal farming practices on the deforested land

These impact the odds of interrupting viral ecology.

Justin Meyer, biologist of University of California San Diego, used this risk map in a public seminar on YouTube.

Habitat destruction forces wild animals to travel farther and nest. Animals then feed and defecate on farms and urban areas instead of insulated wilderness.

Hunters and meat marketers also transport animals out of their native habitat to an urban market in a new region. There, many species that otherwise wouldn’t meet do meet. They also meet more humans. Africa faces similar problems, though to a lesser degree.

Additionally, both regions have bats that scientists have tied to outbreaks. Bats share many viruses that cause deadly outbreaks:

  • In 1967, the Marburg virus first broke out in Marburg, Germany, and traced to imported Ugandan monkeys.
  • In 1976, the Ebolavirus first erupted in Nzara, South Sudan. But scientists first reported the outbreak in villages near the Ebola River, the Democratic Republic of the Congo.
  • In 1994, the Hendra virus killed horses and later humans around Hendra, Australia.
  • Later in 1998, the Nipah virus broke out in several Asian countries. Yet scientists first spotted the virus around Nipah, Malaysia, on pig farms. Then in 2002, SARS coronavirus had its first outbreak in Foshan, China.
  • In 2012, the MERS coronavirus showed up in Saudi Arabia. These viruses continue to break out periodically.
  • The last but already the most notorious was SARS coronavirus-2 of Wuhan, China, in 2019.

Besides these epidemics and pandemics, bats also carry many of the same disease-causing viruses rats do. “Rats with wings” is an apt joke.

Black Flying Fox, associated with the Hendra virus of Australia. Photo courtesy of Andrew Mercer.

Bat, rat, or otherwise, the denser the human and domestic animal population, the more ecologically disturbed the area. The more disturbed the habitat, the greater the risk of spillover.

Besides southern Asia and central Africa, emerging economies like Brazil have a higher risk of spillover. Further, MERS coronavirus testifies that familiar animals in stable environments sprout spillovers.

“We cut the trees; we kill the animals or cage them and send them to markets. We disrupt ecosystems, and we shake viruses loose from their natural hosts. When that happens, they need a new host. Often, we are it.”

David Quammen, author of Spillover: Animal Infections and the Next Pandemic, wrote in the New York Times

Science writers and ecologists have taught about the risk of pandemics for decades. A lot like J. S. Koen, the veterinarian who first suggested animals and humans shared diseases, an ecologist’s opinion can be unpopular.


Runstadler, Jonathan. “The Spillover Effect.” Tufts Now, Tufts University (15 March 2018)., accessed 18 April 2020.

Field, HE. “Bats and emerging zoonoses: Henipaviruses and SARS.” Zoonoses and Public Health, vol. 56, 6-7, (9 July 2009), 278-284. DOI: 10.1111/j.1863-2378.2008.01218.x, accessed 14 April 2020.

Schmidt, Charles. “Why the Coronavirus Slipped Past Disease Detectives.” Scientific American (3 April 2020)., accessed 15 April 2020.

Qui, Jane. “How China’s ‘Bat Woman’ Hunted Down Viruses from SARS to the New Coronavirus.” Scientific American (1 March 2020)., accessed 11 April 2020.

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