According to experts, humans are creating the conditions for the spread of diseases by reducing the natural barriers between virus host animals — in which the virus is naturally circulating — and themselves.

If they say that they should also provide evidence. Okay here it is ...

Tropical forests and intact natural environments teeming with exotic wildlife threatened humans by harbouring the viruses and pathogens that lead to new diseases in humans like Ebola, HIV and dengue.

Ebola, a deadly virus then barely known to humans, unexpectedly spilled out of the forest in a wave of small epidemics. The disease killed 21 of 37 villagers who were reported to have been infected, including a number who had carried, skinned, chopped or eaten a chimpanzee from the nearby forest. Researchers think the virus is animal-borne,  with bats or nonhuman primates (chimpanzees, apes, monkeys, etc.) being the most likely source. Infected animals carrying the virus can transmit it to other animals, like apes, monkeys, duikers and humans.

The virus spreads to people initially through direct contact with the blood, body fluids and tissues of animals. Ebola virus then spreads to other people through direct contact with body fluids of a person who is sick with or has died from EVD. People are eating chimps, apes and monkeys who harbour these viruses, handling their bodies and coming in contact with body fluids in the process. 

Researchers now think that it is actually humanity’s destruction of biodiversity that creates the conditions for new viruses and diseases like COVID-19, the viral disease that emerged in China in December 2019, to arise — with profound health and economic impacts in rich and poor countries alike. In fact, a new discipline, planetary health, is emerging that focuses on the increasingly visible connections among the well-being of humans, other living things and entire ecosystems.

Building roads, dams guest houses in forests is responsible too. We invade tropical forests and other wild landscapes, which harbour 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, recently wrote in the New York Times. “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.”

Human activity drives the emergence of new pathogenic (disease-causing) viruses. As we push back the boundaries of the last wild places on Earth – felling the bush for farms and plantations – viruses from wildlife interact with crops, farm animals and people.

Species that evolved separately are now mixing. Global markets allow the free trade of live animals (including their eggs, semen and meat), vegetables, flowers, bulbs and seeds – and viruses come along for the ride.

Humans are also warming the climate. This allows certain species to expand their geographical range into zones that were previously too cold to inhabit. As a result, many viruses are meeting new hosts for the first time.

Research suggests that outbreaks of animal-borne and other infectious diseases like Ebola, SARS, bird flu and now COVID-19, caused by a novel coronavirus, are on the rise. Pathogens are crossing from animals to humans, and many are now able to spread quickly to new places. The U.S. Centers for Disease Control and Prevention (CDC) estimates that three-quarters of “new or emerging” diseases that infect humans originate in nonhuman animals.

Some, like rabies and plague, crossed from animals centuries ago. Others, like Marburg, which is thought to be transmitted by bats, are still rare. A few, like COVID-19, which emerged last year in Wuhan, China, and MERS, which is linked to camels in the Middle East, are new to humans and spreading globally.

Other diseases that have crossed into humans include Lassa fever, which was first identified in 1969 in Nigeria; Nipah from Malaysia; and SARS from China, which killed more than 700 people and traveled to 30 countries in 2002–03. Some, like Zika and West Nile virus, which emerged in Africa, have mutated and become established on other continents.

Emerging animal-borne infectious diseases an “increasing and very significant threat to global health, security and economies.” According to experts, these zoonotic diseases are linked to environmental change and human behavior. The disruption of pristine forests driven by logging, mining, road-building through remote places, rapid urbanization and population growth is bringing people into closer contact with animal species they may never have been near before.

The resulting transmission of disease from wildlife to humans is now a hidden cost of human economic development. There are just so many more of us, in every environment. We are going into largely undisturbed places and being exposed more and more. We are creating habitats where viruses are transmitted more easily, and then we are surprised that we have new ones.

There are countless pathogens out there continuing to evolve which at some point could pose a threat to humans,” says Eric Fevre, chair of veterinary infectious diseases at the University of Liverpool’s Institute of Infection and Global Health. “The risk [of pathogens jumping from animals to humans] has always been there.” We have created densely packed populations where alongside us are bats and rodents and birds, pets and other living things. That creates intense interaction and opportunities for things to move from species to species”. And Pathogens do not respect species boundaries.

Humans, according to experts, are creating the conditions for the spread of diseases by reducing the natural barriers between virus host animals — in which the virus is naturally circulating — and themselves. Major landscape changes are causing animals to lose habitats, which means species become crowded together and also come into greater contact with humans. Species that survive change are now moving and mixing with different animals and with humans.  The health risks in a natural environment can be made much worse when we interfere with it.

And need we say we are facing the consequences now? wake up world!

Disease ecologists argue that viruses and other pathogens are also likely to move from animals to humans in the many informal markets that have sprung up to provide fresh meat to fast-growing urban populations around the world. Here animals are slaughtered, cut up and sold on the spot.

The “wet market” (one that sells fresh produce and meat) in Wuhan, thought by the Chinese government to be the starting point of the current COVID-19 pandemic, was known to sell numerous wild animals, including live wolf pups, salamanders, crocodiles, scorpions, rats, squirrels, foxes, civets and turtles.

Equally, urban markets in west and central Africa see monkeys, bats, rats and dozens of species of bird, mammal, insect and rodent slaughtered and sold close to open refuse dumps and with no drainage.

“Wet markets make a perfect storm for cross-species transmission of  diseases".

We cannot completely ban this 'wet food ' as they are food source for  poor people. What we can do is good planning, control and preserving our ecology.