This latest bird flu virus is genetically slightly different but overall functionally similar to the H5N1 virus that has killed over 300 people since it first appeared in 2003. Like the H5N1 virus, the newer H7N9 virus is highly virulent, killing 33% of all the persons it infects. And also like the H5N1 virus, the new H7N9 appears to be transmitted to humans only from birds. There have been no documented cases of sustained human-to-human transmission of either of these viruses, at least not yet. The big worry, of course, is that a genetic shift in one of these viruses will make it highly transmissible between humans, much like the common cold. If that happens, the combination of high virulence and rapid human-to-human transmission could lead to a pandemic, meaning that millions of humans might die.
We need to keep an eye on this one.