Originally published as a digital essay for a university course April 22, 2020.

We have been slow to learn the role of virus ecology in human health.

J.S. Koen, a veterinarian and inspector for the U.S. Bureau of Animal Industry in Fort Dodge, Iowa, became unpopular during the Spanish flu outbreak in 1918. He saw “flu-like” symptoms in pigs.

“The similarity of the epidemic among people and the epidemic in pigs was so close, the reports so frequent that an outbreak in the family would be followed immediately by an outbreak among the hogs, and vice versa, as to present a most striking coincidence if not suggesting a close relationship between the two conditions. It looked like the “flu” and until proven it was not the “flu,” I shall stand by that diagnosis.”

J.S. Koen, veterinarian and inspector, U.S. Bureau of Animal Industry

Pig farmers weren’t impressed.

Koen published “A Practical Method for Field Diagnosis of Swine Disease” in American Journal of Veterinarian Medicine in 1919 based on those observations. Today, scientific articles cite him as the first to officially observe zoonotic, or animal-borne, disease.

A few years later, Richard Shope of the Rockefeller Institute of Comparative Anatomy, found pigs had the same flu antibodies that people alive for the Spanish flu had, but people born after the outbreak lacked. Ten years later, other researchers with the U.S. Bureau of Animal Industry also proved Koen’s theory when they transferred mucous samples from isolated sick pigs to healthy pigs, which became sick.

Pigs have similar organ systems to humans. Forensic scientists use pig bodies to study decomposition, and gastroenterologists see the same digestive diseases in humans that veterinarians see in pigs. Many scientists estimate 70% of our immune system resides in our guts. By extension, humans share many of the same infectious diseases as pigs.

The Spanish flu mystery lives to this day. Pigs shared the disease, but infection went both ways. We both suffered outbreaks. Neither of us naturally carried the disease.

Virologist Rob Webster took the investigation one step further in the 1970s. He and a team studied the genetics of viruses in many wildlife species. They found flu viruses everywhere. Birds in particular carried and passed the influenza viruses with little to no disease. There, the viruses were at home. Birds were the viruses’ natural niche. Additionally, modern pig and human flu strains genetically trace to the viruses in several waterfowl species. Likely, the Spanish flu mutated enough to jump from its home to pigs and then to humans, or to humans and then to pigs. This jump is called spillover.

Emory University disease ecologist Thomas Gillespie discusses the origins of Ebola, how it starts and spreads, and why this outbreak may just be the beginning of a much larger problem.

Spillover events happen all the time. Two-thirds of infectious diseases in humans come from animals. Some are self-contained like rabies, where a single infected bite from a raccoon to a person will infect only the one person, assuming the person wasn’t vaccinated and could get infected at all. Each bite counts as a spillover event. On the other hand, major flu outbreaks might originate from one or several spillover events that go on to have widespread human-to-human transmission. With more recent epidemics, we can point to Middle East Respiratory Syndrome, MERS, as a multiple spillover disease. Geneticists traced different virus mutations from separate events of camels to humans. On the other hand, Severe Acute Respiratory Syndrome, SARS, traces to one genetic event from civets to humans.

Point being, the age of disease ecology began. Viruses belong to a web of interactions just like anything else. In their natural setting, they cause little problem for their hosts. Once disturbed, and with the right accident of mutations, viruses spillover to new hosts. Sometimes the new hosts have no problems, but other times they reject the foreign invasion and become sick.

Feature photo courtesy of Kansas State University Research and Extension.


Runstadler, Jonathan. “The Spillover Effect.” Tufts Now, Tufts University (15 March 2018). https://now.tufts.edu/articles/spillover-effect, accessed 18 April 2020.

Hollenbeck, James E. “An avian connection as a catalyst to the 1918-1918 influenza pandemic.” International Journal of Medical Sciences, vol. 2, 2 (15 Mary 2005): 87-90. DOI: 10.7150/ijms.2.87, accessed 20 April 2020.

“Coronaviruses.” National Institute of Allergy and Infectious Diseases. https://www.niaid.nih.gov/diseases-conditions/coronaviruses, accessed15 April 2020.

Andersen, Kristian G et al. “The proximal origin of SARS-CoV-2.” Nature Medicine, vol. 26 (17 March 2020): 450-2. DOI: 10.1038/s41591-020-0820-9, accessed 15 April 2020.


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