Corona and the like: where do they come from?


Published on 8 May 2020

Aerial photo deforestation
Deforestation and increasing agriculture create much more forest edges. Photo: deforestation for rubber and palm oil plantations (© Shutterstock)


Where do pathogens such as the new coronavirus come from? And why do they develop so easily into pandemics? We decided to find out.

Out of nowhere, a virus has gripped the entire world. The economy is in freefall and billions of people are being forced to stay home. All this came as a bolt from the blue.

Or perhaps not? For many years, the World Health Organization (WHO) has been warning about “Disease X”. And Kevin Ariën, virologist at the Institute of Tropical Medicine (ITM), has been telling his students for years that such a pandemic is just around the corner.

Moreover, the “One Health” concept has gradually been emerging since the 2000s: a holistic vision that sees the health of people, animals and ecosystems as a whole, with the very aim of preventing pandemics. ‘We also try to apply this principle at the ITM’, Ariën explains.


The idea that human and animal health are related is clearly not unfounded. Two thirds to three quarters of all new infectious diseases over the last 60 years appear to have come from animals!  Examples include Ebola, AIDS, MERS, SARS, etc. (see below). COVID-19, the disease caused by the current SARS-CoV-2 coronavirus, is also a “zoonosis”: an infectious disease whereby the pathogen spreads from animals to humans.

SARS-CoV-2 probably came from a bat, more specifically a Chinese horseshoe bat. ‘A coronavirus has been found in these bats with a genome that was a 96% match to that of SARS-CoV-2’, says Ariën. ‘But this virus probably first evolved within an intermediate host closer to humans. We also observed that with the previous coronavirus SARS-CoV-1 that caused SARS: it made a detour via civet cats. The MERS virus first passed from a bat to a camelid before reaching humans. For SARS-CoV-2, it may have been a pangolin. These animals are highly sought after in China, because of the healing power of their scales but also for their meat’’.

Possibly the pangolin acted as an intermediate host for SARS-CoV-2. Pangolins are very much in demand in China, for the healing power of the scales, but also as meat (© Shutterstock)

Wildlife market

The “wet market” in Wuhan is singled out in particular as being the site where the novel coronavirus passed on to humans. Just to be clear, wet markets are large markets like the ones we have at home, where you can buy everything: fruit, vegetables, fresh meat and fish, herbs and spices... The really high-risk places are the “wildlife markets” where live game is sold, including badgers, bamboo rats, porcupines, civet cats, scorpions, turtles, bats, you name it. ‘The cages are stacked on top of and next to each other, with the result that faeces and urine falls down from one animal to another. When the animals are slaughtered, blood splatters everywhere. It is a living laboratory for viruses’, that is how zoologist Herwig Leirs (UAntwerpen) recently described it in MO*. Not only the consumption of wild animals, but also the contact with urine, faeces and blood can lead to contamination.

‘These wildlife markets are popular in China and other Asian countries,’ explains Ariën. ‘They are definitely the cause of zoonoses such as bird flu. But the situation with regard to the current coronavirus is a little more complex. With the first 50 cases, there were 14 that could not be linked to that wildlife market in Wuhan. But the market definitely played a role as a catalyst’.

Contact with (wild) animals is ingrained in many cultures. Ariën: ‘In Africa, but also in South-east Asia and Latin America, millions of people depend on (wild) animals for their source of protein, the so-called bushmeat. In these countries, buying a live animal is sometimes the only guarantee of fresh meat. A live chicken, for example, which people will leave to run around before slaughtering it the following week. Of course, this creates the possibility of transmitting pathogens to humans’.

Moreover, wealthier city dwellers also like to eat some bushmeat every now and then. ‘Bushmeat only used to be sold in local communities’, explains Leirs. ‘Nowadays it is also a delicacy for rich city dwellers. The animals are transported over long distances and brought into metropolitan areas, along with the diseases they carry. Once there, these diseases can easily spread.’

a hunter hunts game in Yangambi, DR Congo
For many, "bush meat" is a source of food and/or income. Photo: a hunter hunts game in Yangambi, DR Congo (© Axel Fassio/CIFOR)


Deforestation clearly plays a role. Aries: ‘By felling forests, you chase wild animals from their habitat or you make them group together in smaller biotopes. This brings them in closer contact with each other and consequently increases the risk that pathogens will be transmitted’

Deforestation is increasingly creating a fragmented landscape, also in Africa. What remains - outside the natural reserves - are islands of forest in a sea of farmland, next to areas where farmland encroaches into forests. As a result, there are many more forest edges and people are more likely to come into contact with wild animals such as monkeys. People go into the forests, for example, to look for post-like trees they can use for building.

But the hunt for bushmeat also brings people into the forest more often. A road through a forest provides access, but immediately entails more chance of contact. The same applies to villages that are built in previously wild areas. ‘Indigenous communities have built up immunity over time’, Leirs explains. ‘Nowadays, however, workers travel from further afield to build a road or extract raw materials in a rainforest. These people are not immune. Moreover, they can carry viruses or bacteria with them when they go home’.

Bats and rodents in particular can move relatively easily from deforested areas to areas with human habitation, and adapt to them. For example, bats are sometimes attracted to fruit orchards. ‘This was the case for the Nipah virus in Malaysia (1999). Fruit bats fed on durian and rambutan. Their faeces and urine infected the pigs that roamed about the orchards, and the pigs in turn passed the virus onto humans’, Olivier Honnay, conservation biologist at the KULeuven, recently confirmed in EOS.

Climate change

Climate change also plays a role in the spread of new zoonoses. ‘You can clearly see the impact in diseases transmitted by mosquitoes and other insects’, says Ariën. ‘So-called tropical diseases such as zika, dengue and chikungunya are becoming more common in the Western world. For example, the Asian tiger mosquito has established itself in southern Europe, where the warmer weather means it feels perfectly at home.’

Intensive livestock farming

Intensive livestock farming can also lead to more zoonoses. Herwig Leirs explains: ‘If a limited number of animals are grouped together, a random virus can spread less easily. Suppose I have 5 chickens and a bird flies over and excretes some droppings. One of the chickens could possibly become infected. But a virus does not usually survive too well in a new host species in the early phase, and natural selection does its work, for more efficient multiplication. If the modified virus cannot spread before that first chicken recovers or dies, the infection will disappear. And it only has four chances of evolving further in another chicken anyway.’

‘But if huge numbers of animals are grouped together, it is much more likely that one of them will come into contact with the unknown virus. The virus can also jump over to other animals more easily. And every time the virus multiplies, minor flaws - mutations - can occur in the hereditary material. Most of these flaws lead to less efficient multiplication. But the mutations that provide an advantage for multiplication are quickly selected. The virus therefore has various opportunities to evolve and better adapt to the new host thanks to intensive livestock farming’.

‘As there are more animals, humans are more likely to come into contact with infected animals. Moreover, farmed animals are often less resistant to infection. After all, they have been reared, for example, to produce meat quickly or to produce a lot of eggs or milk. As a result, they are less able to kill a pathogen, they are more easily sick. Antibiotics are used to treat them, but they select resistant bacteria which in turn can cause new diseases’.

Large cities are a fantastic biotope for a virus to spread rapidly. Photo: Mumbai (© Shutterstock)

Travel and mega-cities

Why can a new pathogen grow more rapidly into a pandemic or global epidemic today? Ariën: ‘The last major pandemic, the Spanish flu in 1918 and 1919, killed 20 to 50 million people worldwide. But the world was a completely different place back then. People still primarily travelled on foot, by bicycle or horse-drawn carriage. The global population was 1.7 billion, compared to almost 8 billion today. These days, for international trade and individual travel, hundreds of thousands of planes fly around the world every day. This is why SARS-CoV-2 was able spread from China to the rest of the world in just a few days!’

In addition, more of us are living together in large cities. Ariën: ‘In 1918 the population lived much more dispersed in the countryside. Today there are already dozens of mega-cities, especially in Asia, West Africa and Latin America. Several of these have more than 20 million inhabitants. European cities are not even in the top 25. Large cities are obviously an excellent biotope for a virus to spread rapidly. One of the reasons why the novel coronavirus has been less virulent in Scandinavia is precisely because people still live more dispersed there.’


There is consequently a cocktail of factors that all contribute to accelerating the emergence of a pandemic: unbridled population growth that forces people to be crammed together in cities, and the destruction of nature for reasons of space, poverty and hunger, cultural habits, intensive livestock farming and the need for international trade and travel.

These underlying causes immediately highlight possible solutions. Ariën: ‘We clearly have to take a different approach to our living environment. Animals must be given more space and we should not interfere too much. Otherwise, we will be exposed to more pathogens!’

Olivier Honnay proposes creating buffer zones (such as tree nurseries or reforestation projects) between natural habitats with wildlife and areas with human habitation. This will reduce the risk of contact between humans and animals. By making the supply chains of agricultural products such as soy and palm oil more sustainable, fewer forests will have to be destroyed.

It would also appear to be essential to restrict wildlife markets and wildlife trade (see box "exotic animals"). At the end of January, China already issued a temporary ban on the sale of wild animals. ‘But then you impinge on deep-rooted cultural habits,’ explains Ariën. ‘The same goes for bushmeat in Africa. I think that will primarily be a job for anthropologists, who will have to figure out how to convince people’. Increasing agricultural productivity so that there is less need for bushmeat or providing alternative incomes may help. Bear in mind that in China, for example, at least 14 million people earn their living trading, breeding and hunting wildlife.


The international trade in exotic animals 

The international trade in exotic animals appears to be one of the largest and most complex markets in the world. In an attempt to control this market, the international community drafted CITES, the ‘Convention on International Trade in Endangered Species of Wild Fauna and Flora’, which covers 5,800 animal species and 30,000 plant species that can only be traded under strict conditions.

But it is apparently very difficult to enforce. A lot of illegal bushmeat even ends up in Belgium, in addition to live animals (often reptiles) and illegal meat from non-protected species, bovine animals and smaller livestock. As such, Erik Verheyen, biologist at the Royal Belgian Institute of Natural Sciences and the University of Antwerp, has called for much stricter controls at all international import channels (airports, ports, etc.).

‘In addition, wildlife enthusiasts must be made more aware of the consequences of their choice’, says Verheyen. ‘Some people keep the animals as a hobby, others consider them a delicacy. But in so doing they contribute to the extinction of endangered species and increase the risk of importing pathogens. Although significant financial penalties can be imposed, potential offenders are often unaware of them. Moreover, these penalties are very rarely enforced. Finally, airlines should raise awareness among their customers with posters and brochures.’ Travelers are not allowed to bring illegal souvenirs (see box "advice to travelers").

market with illegal endangered animals in Myanmar
Despite the international treaty CITES, a lot of game continues to slip through the net. Photo: market with illegal endangered animals in Myanmar (© Dan Bennett/Flickr)


Tackling population growth is another avenue that can best be done by emancipating young women. Herwig Leirs highlights our large-scale consumption of animal products. ’Can’t we do with less meat, so that we can grow the animals more extensively? Today we fell forests in order to let cattle graze or sow the cleared land with soy for cattle feed. If we consume less meat, the pressure to destroy forests decreases’.

Early warning

But even in an ideal world where we are much more careful with animals, new diseases cannot be ruled out. That’s why Kevin Ariën is an ardent advocate for prevention. ‘We need to invest much more in early detection and early warning. You can compare it to the buoys that were placed all over the oceans after the tsunami. They can warn long enough in advance that a tidal wave is imminent. We could implement a similar system for virus outbreaks. From a technical perspective, it is feasible. Up until now we have always been far too late’.

‘But then you would need the necessary resources and equipment. And you have to have well-trained people. In the DR Congo, for example, little is invested in public health, and what is invested tends to be for responses to diseases, such as vaccines. Precious little is invested in prevention. The importance of prevention is still not sufficiently appreciated. It is true that billions of euros are needed to make early diagnoses. But the current outbreak will cost us a lot more!’

‘We see it time and again. After SARS, worldwide research into coronaviruses was started. We knew this included some dangerous viruses, but after a few years much of the funding was withdrawn. The sense of urgency had fizzled out. I am sure that after this corona pandemic, we will fall back into old habits. Nonetheless, we need to learn from the current crisis in order to tackle it more effectively in the future’.


Travel advice

People planning to travel to faraway destinations are strongly advised to consult a specialist travel clinic such as the ITM in advance. A list of Belgian centres can be found here. The aim is to provide professional advice to travelers regarding current health risks, specifically for the planned travel destination and type of travel. The health and condition of the traveler may also be taken into account. Based on this, he or she can receive appropriate advice on what to do in the event of illness during or after the trip. The ITG app Wanda is a useful tool in this regard, but does not replace proper consultation before you travel.

It goes without saying that travelers must comply with existing legislation and not take any illegal souvenirs with them, such as endangered animals or plants. Within the EU, you may not bring plants or parts of plants from non-EU countries into the EU, unless accompanied by a recognized "phytosanitary certificate". The exceptions are: pineapple, dates, bananas, coconut and durian.

For more general tips: 17 tips for sustainable travelling