The Chinese New Year is the single biggest travel event on the global calendar; the movement around the country of hundreds of millions of people and the family feasts associated with the multi-day holiday are expected to raise the infection count higher still.
Scientists who monitor influenza are watching China especially closely now.
"For the current period it is important to remain extra vigilant, I would say," says Dr. Sylvie Briand of the World Health Organization, stretching out the first syllable of "extra" for emphasis.
While Briand and other flu experts are worried, they are in a quandary. They know the new virus poses a pandemic risk, perhaps a greater one than any non-human flu virus that has emerged in recent times. Certainly no bird flu virus has infected people so easily and frequently in known history.
In fact the numbers are rising so fast it's tough to keep track of where the count stands. Between the time this article is written and when it is read, the numbers will almost certainly change.
Late Thursday the U.S. Centres for Disease Control's tally was 272 cases and 62 deaths since the first known cases occurred in late February 2013. To put that in context, there have been about 650 cases of H5N1 avian flu over the past decade. In fact, some experts think it is possible that by the time this flu season subsides in China in the late spring, H7N9 may have overtaken H5N1 — in just over a year.
"The march of this virus into other regions in China and other countries will be occurring over the next months and years," says Nancy Cox, who heads the CDC's influenza division.
"So ... even if it doesn't occur by the spring, we certainly will have in time as many cases of H7N9 as we have of H5N1. And I would expect it to take far less time."
But will geographic spread and a rapidly rising case count presage the century's second flu pandemic, the worldwide spread of the virus? Since it can't do it now, it seems H7N9 would need genetic changes to be able to spread easily from person to person. And scientists have absolutely no way to gauge if or when that might happen.
"I can't say whether that mutation will occur tomorrow, in 10 years, or never," Dr. Tom Frieden, director of the U.S. Centres for Disease Control, said Thursday.
That is a fact that is difficult to explain to a public that generally wants clarity, not caveats. Briand, who is director of the WHO's pandemic and epidemic disease department, readily acknowledges the limited state of the current science.
"We know there is a risk, but it is very hard to say (if it is) very low or very high. And most of the time we conclude: OK, it's a middle risk," she says.
Adding to the challenge of assessing the risk and communicating it to the public are long shadows cast by two other flu viruses, H5N1 bird flu and H1N1, the agent responsible for the 2009 pandemic.
The former is deadly to poultry flocks, which it has been ravaging in parts of Asia and North Africa for years. It rarely infects people, but when it does, the outcome is often fatal; roughly 60 per cent of confirmed cases have died.
That lethality earned H5N1 lots of headlines from 2004 to 2007 or so, with even some experts fearing there was an inevitability about the virus's pandemic potential. But the longer it has gone without causing a pandemic, the more people have become inured to H5N1's threat. While there is no science to support the assertion, some believe if the virus were going to cause a pandemic, it would have done so by now.
"You know, for human beings, 10 years is a long time. But we don't know if for viruses it's a long time or not," Briand says.
Instead of a catastrophic bird flu pandemic, a strange swine influenza virus blindsided the world in 2009, causing the first pandemic since 1967. The H1N1 strain was and remains deadly to some — young and middle-aged adults — but the numbers of fatal cases are in relative terms small and H1N1 didn't wreak nearly the amount of havoc the world had feared a pandemic would bring.
In the outbreak's aftermath, the WHO and other public health leaders were criticized as having cried wolf. Some even alleged they'd colluded with Big Pharma to boost sales of flu drugs and pandemic vaccine.
"Coming on the legacy left behind by H5N1 on the one hand and then pandemic H1N1 on the other hand makes it quite difficult to communicate the level of concern that would be appropriate for this one," says Malik Peiris, chair of the virology department in the University of Hong Kong's school of public health.
Risk communications experts Peter Sandman and Jody Lanard have been monitoring developments in influenza and other infectious disease for over a decade.
They suggest this situation has the public health world over a barrel. If they sound alarms about H7N9 and the virus doesn't quickly take off, they'll be dismissed. If they don't sound alarms, and the virus does trigger a pandemic, they'll be pilloried.
"This is a high-magnitude risk whose probability cannot be estimated with any confidence," they said via email.
"And whatever is conveyed, no matter how nuanced, if the outcome goes in the other direction, people will remember the experts as having overly alarmed them, or as having failed to warn them."
The public health authorities and flu experts who spoke out so loudly about the risk of H5N1 a few years ago know that, Lanard and Sandman say. And having seen that virus stick around without causing a pandemic, those authorities and experts themselves may not have the same strong hunches they felt in the early days of the H5N1 outbreak.
"Officials aren't afraid of scaring people; they're afraid of being accused of trying to scare people. And they're afraid of provoking still more cynicism. They don't have much credibility left when it comes to infectious disease warnings, and they're trying to husband what they have," the husband and wife duo say.
Cox makes few bets on flu, and she's not starting now.
"The question for us always is 'Which virus is it going to be?' not 'Will there be another pandemic?'" she says.
"We think that another pandemic is inevitable at some time. Is it going to be H7N9? Or is it going to be another influenza virus that comes out of the blue and surprises us like the H1N1 virus did during 2009? That's the question for us."