As influenzas go, the H7N9 virus at first seemed relatively benign: in late March, the Chinese government reported that three individuals had contracted the illness, and that two had died. The disturbing revelation was tempered, however, by reassurances that human-to-human transmission was highly unlikely, and that Chinese health authorities were taking unprecedented measures to monitor and contain the virus. But four weeks later, the situation has changed: 23 people are now dead, and 122 have been infected across several provinces. Officials with the World Health Organization last week warned that the virus is one of "the most lethal" they've ever seen, and appears to jump easily from birds to humans.

The progression of this virus, it appears, is still far from over. And experts already warn that bird-borne viruses like H7N9 will continue to emerge from China for the foreseeable future — largely because of a unique combination of ecological and cultural factors that make the country a hotbed for deadly avian influenzas.

"I plainly don't think there is a very good way to prevent these influenzas in the future."

"We're all puzzled, in the scientific community, with how to handle the dynamics in China," Marius Gilbert, PhD, an epidemiologist who specializes in the transmission of avian flu at the Free University of Brussels, told The Verge. "I plainly don't think there is a very good way to prevent these influenzas in the future."

Two key elements make China a hotbed for new avian influenza: an exceptional density of people living alongside huge numbers of chickens and ducks, and a thriving live-bird market system. And these live-bird markets are not only a cultural norm, but also integral to a great number of local economies. China is home to a diverse array of avian species, including large populations of both domestic and migratory birds living in the same areas. "Mixing of bird species in small spaces [like markets] provides a great opportunity for the virus to infect different species and to mutate," Tanya Graham, PhD, a veterinary pathologist at South Dakota State University, told The Verge. "Because influenzas are constantly mutating, at some point the virus mutated enough to infect humans."

"Buy a live bird and take it home to kill and cook."

But those markets, which are ubiquitous in many bustling urban areas, probably aren't going anywhere. "The cultural norm is the ability to buy a live bird and take it home to kill and cook," Graham said. Of course, plenty of other countries, including those in Asia and Africa, and even the US, are also home to live-bird markets. What they don't share with China, experts point out, is the combination of myriad avian species and human beings—all living in cramped quarters. "People might suggest that unclean conditions or poor hygiene in these markets are to blame," Gilbert said. "But to me, the density is much more important than the conditions under which these animals are farmed and traded."

The potentially deadly impact of this combination has been seen before, namely in H5N1, another China-borne virus that's killed more than 350 people since 2003 alone. Because H5N1 and H7N9 seem to share similar origins, investigators including Gilbert are already using maps of the H5N1 trajectory to anticipate where H7N9 might turn up next.

"The spectre of rapid worldwide transmission."

"The same places we found high risk of H5N1 spreading, we're finding a high risk with H7N9—because the root causes are [much] the same," he said. One hypothetical map, published in Nature, warns that the "epicenter of current H7N9 outbreaks" is also a major locus of global airport traffic, raising the spectre of rapid worldwide transmission if the virus does indeed start jumping between people. "If individuals can infect one another, this virus will spread around the world in a matter of days," Gilbert said. "It would be extremely hard, if not impossible, to stop that."

For now, public health experts are hustling to unravel the H7N9 virus, confirm its origins, and track its spread—a notably tricky project, because birds infected with H7N9 don't show any symptoms, meaning the virus is tough to accurately keep tabs on. Health authorities are also developing an H7N9 vaccine, though it's likely still six weeks from completion.

Whether or not H7N9 morphs into a global health concern remains to be seen. Regardless, its origins in the markets and farms of China might not be so easy to change—portending the emergence of novel, new viruses in the years to come. "There are a lot of farmers, there are a lot of people in this industry, and there is a [dynamic] of wildlife you can't really change," Gilbert said. "We can make all the recommendations we want, but the ones that might be most effective? I just don't think they'd be implementable."