The H5N1 avian influenza outbreak in 1996, severe acute respiratory syndrome (SARS) in 2003-2004 and now covid-19 were all first detected in China. Some accounts even claim that a precursor to the 1918 influenza pandemic, the worst in modern history, first appeared in China.

Why do so many highly infectious diseases appear to start their deadly spread in China?

Many recent articles have honed in on the policy errors of China’s authoritarian political system. As the novel coronavirus began to sweep across the United States, President Trump sought to directly implicate Beijing by referring to “the Chinese virus,” instead of its technical designation, SARS-COV-2.

But politics and policy aren’t the only factors behind pandemic disease outbreaks. Research shows that geography and economics play key roles, too. It’s important to understand these different dimensions of viral outbreaks to prevent future pandemics emanating in China — and elsewhere.

So, how do new diseases start and spread? Here’s what you need to know.

1. Geography isn’t destiny — but it explains a lot

In principle, viral outbreaks can happen almost anywhere. A decade ago, H1N1 swine flu likely arose from the mixing of viruses from the pig populations of several countries, including Mexico, Europe and the United States. Even more strikingly, another candidate for the origin of the 1918 influenza is Kansas.

In practice, though, novel viruses tend to arise disproportionately from “pandemic hot spots” — places with high human population densities, high levels of biodiversity, large animal populations that can harbor and incubate new diseases, and patterns of regular interaction between animals and humans. This provides viruses with opportunities to jump from one species to another — a process scientists call “host-switching.”

The risk of viral disease outbreaks also appears to be higher in regions experiencing rapid changes in land use. Until the recent few decades, most major land-use changes, whether clearing forests for farmland or building new roads and railways, tended to take place in mid- and high-latitude regions with lower biodiversity. But these changes now are increasingly concentrated in the tropical and subtropical regions of the developing world — places where biodiversity is highest.

And where there are more animal species, you’re sure to find more viruses. As humans push further and deeper into previously sparsely populated tropical forests and similar habitats, this creates more opportunities for viruses to switch from animal to human hosts. And as it turns out, the speed and scale of urbanization in China has been without precedent in human history.

Parts of southern China, with its high biodiversity levels, dense human populations, rapid development and sprawling live-animal markets, have long fit the bill for becoming potential pandemic hot spots. In the case of SARS, scientists believe the disease originated in bats in the caves of southwestern Yunnan Province and was transmitted to humans via wildlife traders.

But China isn’t the only country prone to these viral transmission pathways: there are plenty of other at-risk regions. The puzzle persists: Why have several highly infectious viral diseases started in China?

2. Global integration is good — except perhaps for viral transmission

What makes southern China different from other at-risk areas is its extensive integration with the rest of the world. Once isolated even from other parts of China, potential pandemic hot spots in south China are now just a few hours’ journey from almost anywhere else in the country thanks to plentiful air links and high-speed rail connections, and the world’s most extensive highway network. The number of international flight connections between China and the rest of the world, meanwhile, more than tripled within the past two decades. Guangzhou, southern China’s largest city, became home to the world’s 13th-busiest airport, with passenger numbers growing by approximately 6 percent a year.

In addition, south China is at the center of some of the world’s most extensive diaspora networks, and it has traditionally accounted for an outsized share of Chinese emigrant populations. U.S. Census data, for example, recorded nearly 50,000 more speakers of the southern Cantonese dialect of Chinese living in the United States than Mandarin speakers, although Mandarin speakers comprise a much greater share of China’s national population.

These networks have played a critical role in making southern China one of Asia’s wealthiest and most dynamic regions by channeling overseas investment and helping to create a major hub for global trade and commerce. But extensive economic, infrastructural and social integration with the rest of the world has a downside: potentially faster exposure to new viral outbreaks. Within days of the arrival of the SARS virus in Hong Kong in early 2003, it had spread to places like Toronto and, shortly afterward, even Gainesville, Fla. It’s south China’s dense integration with the rest of China and the world that distinguishes the region from other potential pandemic hot-spot regions.

What does all this mean for the next pandemic?

In many ways, China is on the leading edge of several trends that are increasing the risk of new viral disease outbreaks around the world. First, rapid development in highly biodiverse regions brings more people into contact with exotic animals and the viruses that lurk within them. Second, ever-more-extensive transport, trade and communications links continue to shorten the distances between a potential pandemic hot spot and virtually any other point on the globe.

One lesson from China’s experience is already apparent: Epidemic surveillance is a true global public good, critical to protecting not just lives but entire economies. Going forward, we can expect China and other countries to focus on two big questions as they seek to prevent future pandemics and deal with the consequences of the current one.

First, will covid-19 force a rethinking of the balance between information control and exchange for habitually secretive states like China? Second, does the risk of rapid viral disease transmission permanently dent the appeal of globalization and global integration? Even if, as seems likely, the answers are no, the very fact that people pose these questions is testament to just how much dealing with a succession of viral diseases has changed the world.

Zeke Emanuel is vice provost for global initiatives and Diane v.S. Levy and Robert M. Levy University Professor at the University of Pennsylvania.

Scott Moore is director of the Penn Global China Program at the University of Pennsylvania and is currently writing a book on China’s role in providing global public goods.

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