In China, nine people have been sickened and three have died from a new strain of H7N9 bird flu. Early scientific examination of the viral strain responsible shows that this avian influenza may be running relatively silent in birds, making it harder to track.

It's the first time this particular subtype of influenza A has been seen in humans, so health officials are taking the cases seriously. A World Health Organization outpost in Beijing has already sequenced the genetic code of viruses taken from three of the patients and has posted them online for researchers to study. (The flu virus genome, made up of RNA, consists of just eight genes, two that code for surface proteins and six that code for internal proteins.)

Results from the earliest analyses of this flu’s genetic code are slightly worrisome. There’s a possibility that this particular virus can travel between birds without causing noticeable serious illness.

“That would make the virus difficult to monitor even as it causes serious disease in humans,” Declan Butler wrote for Nature. “Should the virus become established in birds, regular human infections might then occur — providing opportunities for it to adapt better to humans, and ultimately spread between them, potentially sparking a pandemic.”

Any time an influenza virus crosses species it is a cause for concern, because the new host will not have developed immunity to that particular strain. The virus behind the SARS epidemic 10 years ago is thought to have jumped to humans in China from any number of animals, including imported civet cats.

But there is good news: There's still no evidence yet that this H7N9 strain can be passed from human to human. Some viruses that jump from animals to humans can’t be transmitted between people – and such “dead end” infections are not likely to cause pandemics.

“Bird flu” is a collective term for any kind of influenza A virus adapted to birds. Until now, just four subtypes have been shown to cause serious illness in humans: H7N3, H7N7, H9N2, and H5N1. Last year, scientists said that another kind of avian flu, H3N8, seems to have jumped to seals and may have been responsible for killing 162 seals in 2011.

Most news reports in recent years about “bird flu” refer to a highly pathogenic H5N1 strain, which is still spreading throughout the globe after originating in Asia. H5N1, however, causes severe sickness in birds, making it easier to spot the virus and cull the poultry that fall ill.

The majority of humans that have been sickened with H5N1 bird flu came into contact with live or dead poultry. It’s still unclear how the victims of the new H7N9 virus were exposed. To avoid exposure, the WHO recommends adhering to basic hygenic practices: wash your hands before and after you eat or prepare food, after you use the toilet or handle animals or animal waste, and frequently when someone in your household is sick.

Subtypes of flu virus get their name from two proteins they carry on the surface: hemagglutinin and neuraminidase. There are 17 kinds of hemagglutinin and 10 different kinds of neuraminidase. So, this latest avian flu strain carries the H7 variety of hemagglutinin and the N9 variety of neuraminidase.

Governments and scientists identify specific strains with a set formula: influenza type (A, B, or C)/ originating host species (if discovered in a non-human animal)/ geographic origin/ strain number/ year isolated; if the flu is an A-type virus, the subtype is then given in parentheses.

So, one strain of avian flu might be identified as A/duck/Alberta/35/76 (H1N1).

With the new strain of avian flu, the most important thing to find out will be whether the virus can jump from person to person. The WHO is also investigating any possible connection between the flu cases and the discovery of more than 16,000 pig carcasses dumped in the rivers near Shanghai, but at the moment, there’s no evidence to draw a line between the two events.

University of Reading virologist Ian Jones told Reuters that it’s too soon to get alarmed about the new bird flu.

“Of course we need to take account of these cases and follow up the contacts and so on, but I think that's where it rests at the moment," Jones told Reuters. "It's far too soon to assume this is the start of something."