- A coronavirus outbreak has killed 25 people and infected more than 830 in China.
- It has spread to eight other countries, including the US.
- The virus originated in Wuhan, a dense city in central China that serves as a major transportation hub and had markets selling live animals.
- Public-health experts say the combination of these conditions was a recipe for a viral outbreak.
- Visit Business Insider's homepage for more stories.
When Chinese authorities first reported an outbreak of a new coronavirus in Wuhan, China, many public health experts had a similar reaction: It was only a matter of time.
"This has always been waiting to happen," Adrian Hyzler, the chief medical officer at Healix International, which offers risk-management solutions for global travelers, told Business Insider. "Wuhan is such a big city. It's 11 million people."
Hyzler said a few specific conditions put Wuhan at especially high risk of a disease outbreak: the city's density, its role as a transportation hub, and the presence of live-animal markets there.
As of Thursday, the virus (officially known as 2019-nCoV) has killed 25 people and infected more than 830. It has spread to at least eight other countries: the US, South Korea, Japan, Taiwan, Saudi Arabia, Singapore, Thailand, and Vietnam. Scientists confirmed that it can pass from human to human, and early research indicates that the virus may have jumped from bats to snakes first.
"The people who are likely to die at first will be people who have other illnesses," Hyzler said. "But as it spreads, it'll pick up more people like flu does."
Wuhan's large, dense population can lead disease to spread
Wuhan is sometimes known as "China's Chicago" because of its proximity to rivers and lakes, but it's about four times bigger. Its population exceeds that of New York City by about 2.6 million.
In terms of land area, the city is more than twice the size of Shanghai and eight times the size of Hong Kong. But people there live tightly clustered within the city limits. Wuhan is the most densely populated city in central China.
Crowded conditions can increase the likelihood of people transmitting infectious diseases, research from the University of Geneva has shown. That's because people in close proximity are more likely to touch, shake hands, or transmit the virus through the air by coughing or sneezing.
On top of that, Wuhan is a major Chinese transportation center. The city is one of China's most important railway hubs, with multiple stations connected to numerous rail lines, including high-speed rail lines that connect to Shanghai and Guangzhou — cities with populations of 24 million and 13 million people, respectively.
The city's Tianhe International Airport serves more than 20 million passengers annually. Wuhan is also an important port city on the Yangtze River, handling more than 1.5 million containers a year. This provides even more channels through which disease can spread.
Wuhan officials have effectively quarantined the city by shutting down transportation, including buses, ferries, and metros. Trains and airplanes coming in and out of the city were also halted.
Its 11 million residents have been told not to leave, barring special circumstances.
The outbreak's timing is especially worrisome, since hundreds of millions of people are expected to travel to and within China for the Lunar New Year celebration, which begins Saturday and lasts until February 8.
"This couldn't have happened at a worse time for Wuhan," Hyzler said.
Live-animal markets encourage the transmission of viruses
The Wuhan virus likely originated at a wet market, where locals and visitors shop for meat and produce among outdoor stalls. The markets put people in close contact with live animals, such as chickens, pigs, donkeys, sheep, reptiles, and fish.
These conditions can make it easy for a virus to jump from an animal to human.
According to the Centers for Disease Control (CDC), more than 75% of emerging diseases originate in animals. Severe acute respiratory syndrome (SARS) originated in a wet market, as did the H7N9 and H5N9 bird flus.
Authorities in Wuhan banned the trade of live animals at wet markets on Wednesday.
"Ever since SARS, there have been calls for improving or closing down live animal markets in China and other countries, particularly in Asia," Eric Toner, a scientist at the Johns Hopkins Center for Health Security, told Business Insider. "There's an awful lot we don't yet know, but I think it is fair to say that live-animal markets are a threat not just to the people who work in them, but to public health more generally."
Hyzler said close contact between people and live animals in Wuhan is a recipe for a pandemic (though the current outbreak is not considered a pandemic).
"Everyone in public health is waiting for another pandemic," Hyzler said. "But in countries that aren't looking for it, you're not going to find it until it spreads."
Contagious people might have left Wuhan before the quarantine
Viruses in the coronavirus family also cause pneumonia, the common cold, and SARS, which affected about 8,000 people and killed 774 in China between 2002 and 2003.
Symptoms of this new coronavirus include a fever, chills, headaches, and a sore throat.
"With SARS, people were only contagious when they were very, very sick, which is why it spread mostly in hospitals," Toner said. "What we don't yet know is to what extent this disease can be transmitted by people who are only mildly ill."
Scientists are still learning about the virus' incubation period — the time that passes between when a patient gets infected and when they start displaying symptoms. During that period, people can be contagious without realizing it. They can also pass through airport screenings without triggering any red flags.
Academics from Imperial College London have estimated that the true number of infected people might be between 4,000 and 9,700.
"Being a coronavirus, it certainly has the potential to spread to countries all over the world," Hyzler said. "This is going to spread to anywhere where there's travel out of Wuhan."
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