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The coronavirus spreading in China and the SARS outbreak of 2003 have two things in common: Both are from the coronavirus family, and both started in wet markets.
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At such markets, outdoor stalls are squeezed together to forming narrow lanes, where locals and visitors shop for cuts of meat and ripe produce. A stall selling hundreds of caged chickens may abut a butcher counter, where uncooked meat is chopped as nearby dogs watch hungrily. Vendors hock skinned hares, while seafood stalls display glistening fish and shrimp.
Wet markets put people and live and dead animals - dogs, chickens, pigs, snakes, civets, and more - in constant, close contact. That makes it easy for a virus to jump from animal to human.
On Wednesday, authorities in Wuhan, China - where the current outbreak started - banned the trade of live animals at wet markets. The specific market where the outbreak began, the Huanan Seafood Market, was shuttered on January 1. The coronavirus that emerged there has so far killed 17 people and infected nearly 550.
"When you bring animals together in these unnatural situations, you have the risk of human diseases emerging," Kevin Olival, a disease ecologist and conservationist at the EcoHealth Alliance, told National Geographic. "If the animals are housed in bad conditions under a lot of stress, it might create a better opportunity for them to shed virus and to be sick."
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Coronaviruses are zoonotic diseases, meaning they first spread to people from animals. In the case of SARS, and likely this Wuhan coronavirus outbreak as well, bats were the original hosts. The bats then infected other animals, which transmitted the virus to humans.
The Huanan Seafood Market in Wuhan closed January 1, after it was found to be the most likely starting point for the outbreak of this coronavirus, also called 2019-nCov.
A 61-year-old man was the first person to die from the virus. According to Bloomberg, he was a regular shopper at the Huanan wet market, which sold more than seafood.
Reports indicate that before the Huanan market closed, vendors there sold processed meats and live consumable animals including chickens, donkeys, sheep, pigs, camels, foxes, badgers, bamboo rats, and hedgehogs, as well as reptiles like snakes.
Wet markets like Huanan are common around China.
On Wednesday, Wuhan authorities banned the trade of live animals at wet markets.
Police in Wuhan began conducting checks to enforce the rule among the city's roughly 11 million residents, the BBC reported, citing state media reports.
This type of intervention could help stop to spread of zoonotic viruses like the Wuhan coronavirus.
"One intervention, which is fairly simple, is just reducing the wildlife trade and cleaning up the wildlife markets," Olival told National Geographic. "Cutting back the wildlife trade has a win-win effect of both protecting species that are harvested from the wild and of reducing spillover of new viruses."
The close proximity of shoppers to stall vendors and live and dead animals in wet markets make them prime breeding grounds for zoonotic diseases.
Between 2002 and 2004, SARS killed 774 people across 29 countries. It originated in Guangdong's wet markets.
In the case of SARS, humans caught the virus from weasel-like mammals called masked palm civets.
But the civets weren't the original hosts of the disease.
Researchers figured out that SARS originally came from a population of bats in China's Yunnan province.
"Coronaviruses like SARS circulate in bats, and every so often they get introduced into the human population," Vincent Munster, a virologist at the Rocky Mountain Laboratories, told Business Insider.
Bats can pass along viruses in their poop: If they drop feces onto a piece of fruit that a civet then eats, the civet can become a disease carrier.
Experts haven't yet identified the animal species that enabled the Wuhan coronavirus to spread to people.
"There's an indication that it's a bat virus, spread in association with wet markets," Munster said. "But we don't know which animal species is the amplifier, or intermediate host."
According to a group of scientists who edit the Journal of Medical Virology, the culprit in this case could be the Chinese cobra.
Scientists in China have figured out the genetic code of the Wuhan coronavirus. When researchers compared it to other coronaviruses, they found it to be most similar to two bat coronavirus samples from China.
But further analysis revealed that the genetic building blocks of the Wuhan coronavirus more closely resemble that of snakes. According to the researchers, the only way to be sure of where the virus came from is to take DNA samples from animals sold at the Huanan market and from wild snakes and bats in the area.
The H7N9 and H5N9 bird flus — also zoonotic viruses — were likely transmitted to humans in wet markets, too.
According to the World Health Organization, people caught those bird flus via direct contact with infected poultry in China. The diseases killed a collective 1,000 people globally.
Bats and birds are considered reservoir species for viruses with pandemic potential, according to Bart Haagmans, a virologist at the the Erasmus Medical Center in Rotterdam, Netherlands.
"Because these viruses have not been circulating in humans before, specific immunity to these viruses is absent in humans," Haagmans told Business Insider.
"There have been plenty of eminent epidemiologists predicting 'pandemic X' for a number of years now," Adrian Hyzler, the chief medical officer at Healix International, told Business Insider.
These pandemics "are more likely to originate in the Far East because of the close contact with live animals [and] the density of the population," Hyzler added. His firm offers risk-management solutions for global travelers.
The Wuhan coronavirus outbreak isn't currently considered a pandemic, however.
Since December, 544 cases of the Wuhan coronavirus have been reported across six countries, including the US. Symptoms include sore throats, headaches, and fevers, as well as pneumonia-like breathing difficulties.
Haagmans said one of the challenges in containing this outbreak is that a substantial portion of infected people show only mild symptoms.
These individuals "may go unnoticed in tracing the virus and fuel the outbreak," he said, adding, "it seems that this actually may be the case now."
