Corona has taught most of us humility: although we live in a high-tech world with Mars rockets, nanorobots and artificial intelligence, we failed to quickly contain or disarm the virus. On the contrary, for months we had to resort to measures from the Middle Ages. People were quarantined just as in the 14th century during the black plague. Or they were isolated just like those suffering from smallpox in the 16th century. People suffered and died as alone as they did 500 years ago.
The development of several vaccines late last year fueled hopes that a return to normalcy would soon be possible. However, despite substantial progress in some countries, most epidemiologists have since had to abandon the goal of herd immunity. The virus changes too quickly (keyword: immune escape), and too many people are skeptical about vaccination. Meaning there will always be local outbreaks in the future, which can affect vulnerable groups particularly hard. The president of the World Medical Association, Frank Montgomery, recently said that he expects the virus to be with us for “generations to come”.
So one way or another, we will have to live with Corona. But why does humanity keep having to suffer the scourge of epidemics and pandemics which we can only manage with great difficulty and at great human cost? To better understand that, we need to take a closer look at the processes by which infectious diseases develop.
The genesis of infectious diseases
60–70 percent of all new infectious diseases occurring in humans originate from animals (1). The process of this “jump” from species to species is called zoonosis. Viruses are normally adapted to a host species and harmless for others. But as viruses are constantly replicating, and errors or random changes in the genetic information occur, a mutation can cause the virus to migrate. Such jumps are more likely to happen the less proximity there is between humans and animals. And in a new host, the virus usually encounters a completely naive immune system that has hardly any adequate defense mechanisms.
Transmission to humans can occur through direct contact with living or deceased animals, through consumption of (raw) animal products, as in the case of salmonella infections, or via an intermediate host, often ticks or mosquitoes. Well-known examples include tick-borne borreliosis or early summer meningoencephalitis, and mosquito-borne malaria as well as zika virus infections.
How zoonoses become epidemics
In the past, it’s often been periods of global interconnecting that have led to epidemic outbreaks. Along trade routes such as, say, the Silk Road the pathogen Yersinia pestis spread. The ensuing black plague killed more than 25 million people worldwide between 1346 and 1353, a third of Europe perished.
The smallpox virus — probably transmitted from rodents — was brought to the American continent from Europe by settlers in the 16th century, where it caused devastating epidemics among Native Americans and wiped out entire tribes. At that time, too, attempts were made to contain the pathogen by means of isolation in “pox houses” *.
The Spanish flu, a highly contagious influenza virus new at the time, traveled the other way. The virus originated on a pig farm in the United States and was carried to Europe by an American farmer who was a soldier in the U.S. Army fighting in World War I. It claimed between 20 and 50 million lives in the two years following World War I.
Beginning in 1959, the HI-Virus spread through Africa. Consistent scientific evidence suggests that it took its route of transmission from apes to Great apes (hominids) and to humans when hunters of wild apes first became infected by eating the meat and coming into contact with body fluids.
The frequency of outbreaks is increasing
But whereas in the past there were often centuries between epidemics and pandemics, the number of new infectious diseases in humans has increased rapidly in the last 30 years (1).
The 1990s were marked by outbreaks of Hanta virus (1992), Hendra virus (1994), Nipah virus (1998), and West Nile virus (starting in 1999). The SARS-CoV 1 outbreak in Asia between 2003 and 2004 heralded a new spate of severe respiratory infections.
Swine flu, an H1N1 influenza virus, claimed more than 200,000 lives between 2009 and 2010. In 2012, a new coronavirus, Middle East Respiratory Syndrome (MERS), was discovered in the Arabian Peninsula which — similar to SARS-CoV 1 — can lead to cases of severe pneumonia. It is transmitted to humans by dromedaries. Finally, the Ebola virus causes epidemics in West Africa between 2014 and 2016, killing over 11,000 people (3).
A fertile ground for pandemics
Factors that have led to the spread of more and more new pathogens over the past three decades threaten to further accelerate this trend. Globalization and an overall increase in mobility cause diseases to spread rapidly across borders and continents.
Due to rapid population growth and the clearing of forests for agricultural land animals from previously isolated regions come into contact with humans. The destruction of ecosystems such as the tropical rainforest has led to the habitat of many animals becoming scarce. These animals can carry viruses which are completely unknown to our immune system.
At the same time, climate change means that intermediate hosts can migrate to new regions where humans are not immune, making new unprotected hosts available to the virus. In addition, humans are deliberately taking rare species out of their natural habitat for wildlife trafficking and bring them to markets such as Huanan Seafood Market in Wuhan — which is considered as a possible ground zero of SARS-CoV 2.
When the first cases appeared there in late 2019, scientists analyzed the group of infected people: 49 of the 99 first infected people stayed at the market before their infection, and 47 of them worked at the market as vendors (2). The Huanan Seafood market is a so-called wet market where live or freshly slaughtered wild animals are sold.
Coronaviruses closely related to SARS-CoV 2 were found in bats. It is quite possible that transmission could have occurred from bats sold alive or dead at the market to market visitors.
Unprepared for the next Corona-event
So while Covid-19 is still ongoing, we already need to prepare for the next set of infectious diseases. “We’re going to get more pandemics. I don’t think there’s any doubt about it”, says Dr. John Bell, Regius Professor of Medicine at Oxford University. He also expects future pandemics to have much higher mortality rates. But while many people are (understandably) eager for this pandemic to end (for now), he and his colleagues believe we must get ready for future threats. “The globe is wholly unprepared for dealing with global health crises of any kind.” (4)
This pandemic has shown us how difficult it is to deal with emerging infectious diseases. Testing and vaccinations cannot adequately compensate for antiquated isolation measures. At the same time, we must expect new pathogens and more frequent outbreaks in the future. Therefore, it is imperative to fundamentally improve quarantine and protective isolation measures with humane innovations as well.
*Pox, variola or variola (Latin variolae), also called smallpox disease, refers to an infectious disease that is dangerous and life-threatening to humans and is caused by smallpox viruses (Orthopoxvirus variolae).
(1) Jones, K., Patel, N., Levy, M. et al. Global trends in emerging infectious diseases. Nature 451, 990–993 (2008). https://doi.org/10.1038/nature06536
(3) Chen N, Zhou M, Dong X, Qu J, Gong F, Han Y, Qiu Y, Wang J, Liu Y, Wei Y, Xia J, Yu T, Zhang X, Zhang L. Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study. Lancet. 2020 Feb 15;395(10223):507–513. doi: 10.1016/S0140–6736(20)30211–7. epub 2020 Jan 30. PMID: 32007143; PMCID: PMC7135076.