An influenza A virus called H7N9 exploded onto the global infectious-diseases radar on April 1 when the World Health Organization revealed China had found three people infected with a new form of bird flu. Since then 77 cases (as of 1 P.M. EDT, April 16) have been confirmed, and the virus has spread from China’s largest megalopolis, Shanghai, and several surrounding provinces to the capital, Beijing, more than 950 kilometers to the north. And cases have proliferated at a startling pace: Infections with H7N9 have already outstripped the total number of H5N1 bird flu cases seen in all affected countries last year.

Whereas the new virus doesn’t seem quite as deadly as H5N1 (fatal in about 60 percent of known cases), it still packs a scary punch. To date 16 people have died from H7N9, and many others remain in critical condition. So far, at least, mild infections from this new strain are in the minority.

“This is very, very severe disease—and rapidly progressive,” Timothy M. Uyeki, an influenza epidemiologist at the U.S. Centers for Disease Control, said last week after Chinese doctors published a report in The New England Journal of Medicine on the first three cases. “It’s similar to H5N1, but it’s a little hard to make comparisons based upon three cases and limited data.”

The new virus has flu experts on high alert. Like H5N1, the H7N9 avian virus doesn’t appear to spread from person to person; researchers think it is jumping to humans from infected birds. But the latter has already acquired some genetic changes that suggest it is at least partially adapted to spreading among mammals. For instance, researchers have found a change at the receptor-binding site that dictates which types of cells it can infect—those found in birds or those found in the human upper airway, where seasonal flu viruses attach and invade. H7N9 is gradually becoming more adept at invading mammalian airways.

The World Health Organization is watching the situation closely. Given the mutations in the virus and the rapid accrual of cases, the agency is preparing for the worst. “When you put it all together, this is a quite serious signal for us. And there’s nothing which has lessened the seriousness of that signal over the last few weeks,” says Keiji Fukuda, WHO assistant director general of health security and environment and the agency’s senior flu expert. The Geneva-based organization is working to determine what needs to be in place if H7N9 vaccines must be made and used as well as how quickly antiviral drugs could be deployed if the need arises.

The big question: Will H7N9 be like H1N1, the swine flu pandemic that took off with an explosive start in April 2009? Or will it be like H5N1, a virus that occasionally jumps from poultry to humans, posing a lingering threat—one which has yet to morph into a major public-health threat, but never goes away either.

It is too soon to tell which path the new virus will take. In fact, H7N9 has some qualities that may put it in a category all of its own. Topping the list is the virus’s low pathogenicity (“low path,” in flu-speak) in poultry. It doesn’t kill chickens and other domestic birds; at this point, it doesn’t even appear to make them sick. H5N1 is a high-path avian influenza virus—it wipes out poultry flocks. High-path viruses are hard on farmers, but dead birds at least help authorities locate the virus so they can attempt to eradicate it. In contrast, H7N9 is virtually invisible in the environment. Scientists from China’s national avian flu reference laboratory have found the virus by testing chickens and pigeons in live animal markets, but authorities still do not know how far this virus has spread or how many types of animals can be infected.

“This is a really, really going to be much more difficult to deal with," says David Halvorson, an avian influenza expert and professor emeritus at the University of Minnesota. “You don’t have this mortality to help guide you, so you have to run around looking for a virus in healthy [animal] populations. And then once you find it, what are you going to do? Chances are you’re going to have it all over the place. It’s way worse than a high-path virus."

A senior official at the Paris-based World Organization for Animal Health agrees. The world has never seen an H7 influenza virus like this H7N9, says Alejandro Thiermann, who noted most previous H7 infections in humans have been mild, often only causing conjunctivitis, or pink eye.

“There are no clues to help us,” Thiermann says. “The only thing is this: Don’t take precautions just based on previous experiences because more likely this is going to be again uniquely different. I hope that it mutates and disappears and just becomes a sporadic low-pathogenic virus somewhere…. [But] we cannot lower the guard, because we cannot treat it like any other low-path [virus] due to the fact that it affects humans so aggressively.”

The one silver lining? Thanks to H5N1 and H1N1, researchers and public health officials investigating this new avian flu and planning contingency responses are well practiced. Says Fukuda: “We’re a lot more experienced. We’re just a lot better prepared—I think.”