The continued existence of wildlife markets, which are considered potential breeding grounds for the spread of harmful viruses, means it's just a matter of time before the world is hit with another deadly pandemic, some scientists suggest.

"If we do not deal with this, there is nothing to say that we could not in eighteen months' time have another outbreak, and it could be worse," said Kerry Bowman, an assistant professor and bioethicist at the University of Toronto's Dalla Lana School of Public Health.

Scientists believe the novel coronavirus that causes the disease COVID-19 emerged from one of these wildlife markets — also known as "wet markets" — in the Chinese city of Wuhan, possibly through an infected bat.

Bats are just one of the animals that are sold at these markets, where customers come to purchase domestic livestock and wildlife, including pigs, chickens, civet cats, bamboo rats, porcupines and pangolins.

But Bowman said shutting down such markets may prove extremely challenging, as these cultural practices date back thousands of years and have become part of a multi-billion-dollar global industry.

Spillover event

Bowman said the main concern with these markets is a spillover event, when viruses transfer from one species to another and then cross over to humans.

On very rare occasions, humans will transmit that virus from one person to another, which is what occurred with Severe Acute Respiratory Syndrome (SARS) and, by all indications, is happening with COVID-19.

WATCH | An explainer on where the coronavirus came from

It started with animal to human transmission. But the novel coronavirus is now spreading between people. 0:25

"Mathematically, this is not a common occurrence. But if you actually wanted to create a laboratory-like experiment to design the conditions for a spillover event, you would create the kind of wildlife market that you have in China," Bowman said.

In a recent video message about COVID-19, famed primatologist Jane Goodall warned that the close relationship between people and wild animals in these markets "has unleashed the terror and misery of new viruses."

SARS may have spread to humans through wild mammals. The Ebola epidemics in west and central Africa are thought to have originated from bats, while Middle East Respiratory Syndrome (MERS) is suspected to have come from camels.

All of these infected animals were in wildlife markets, where every day millions of people around the world still get their food.

Not just in China

Bowman emphasized that China is not the only culprit, nor is Asia "the only continent in the world that has this challenge."

"But it's particularly tenacious in the Far East, and extends into Vietnam, a lot of Southeast Asia, Indonesia," he said.

Civet cats wait for food at a farm in Liuan, China. (China Photos/Getty Images)

In February, China announced a ban on the trade and consumption of wild animals. But it also imposed tough restrictions following the SARS outbreak in 2003, only to see the industry slowly re-emerge. Many environmentalists say such bans contain loopholes.

The selling of wild animals accounts for "a significant portion of the economy. And it has created an economic opportunity for a lot of people," Bowman said.

There are different estimates, but Bowman said the size of the global wildlife trade is pegged at somewhere between $7 billion to $23 billion US a year.

Animals in unsanitary conditions

Bowman, who conducts research on this issue, said when he last visited a market in Wuhan, he counted 57 species of animals, about two-thirds of them wild, mostly of Asian origin. Cages were stacked on top of each other, in unsanitary conditions.

He said operators had "high-powered hoses that are blasting around urine, feces, blood from one cage to another to another to another."

In terms of the general concepts of infectious diseases, wildlife markets are "a perfect opportunity for the mixing of bacteria and viruses as well as transmission to other groups," said Jason Stull, assistant professor at the University of Prince Edward Island's Atlantic Veterinary College.

Not only that, but stress and malnutrition reduce the immune system of animals and potentially exacerbate this problem, Stull said. For example, an animal under duress may be more likely to shed higher amounts of virus.

"All of these things likely can contribute to movement back and forth of diseases," Stull said.

Many infectious diseases linked to wildlife

According to the EcoHealth Alliance, a New York-based organization that conducts scientific research into emerging infectious diseases, about three-fourths of all such diseases are somehow linked to wildlife.

William Karesh, executive vice president for health and policy at the EcoHealth Alliance, said the current coronavirus outbreak was likely spread in two possible ways. It could have been a wild animal being sold in the market that contaminated the market. It's also possible that a vendor in the market was infected somewhere else and then infected their customers.

The animals that end up in the market are coming from two places — hunted in the wild or bred on farms.

WATCH | How bats likely spread the novel coronavirus

Bowman said there are deep cultural roots with this industry — thousands of years of tradition of eating wild animals. As well, the animals are used for traditional Chinese medicine, luxury goods and the pet trade.

"What's really changed is that this has gone from occasional domestic use with emerging populations in combination with the burgeoning wealth to a massive commercial enterprise," Bowman said.

Focus should be on education

Karesh said ending these practices will take time, likely generations, and can only be done through education and helping countries improve their food systems.

He suggested that instead of banning all wildlife trade, countries should focus on those animals that are more likely to have viruses that can be transmitted to humans — like rodents, bats and non-human primates.

He said the international community must come to grips with the growing and unsustainable use of wildlife, or we will "continue to see pandemics."

"There are three to five emerging diseases every year, and only by luck and the grace of God ... they don't turn into pandemics each time."