In its native state, according to the CDC, the H5N1 flu virus is highly contagious and is especially deadly to birds. Fortunately, few people have contracted this strain of flu as it's quite deadly in people as well. Luckily, there have been very few cases of transmission of the virus between people, indicating that the protein the virus variant uses to bind to human cells is quite weak. That does not appear to be the case, however, for a mutant version of the virus created by Japanese scientists last year.
In that effort, the Japanese team mutated the hemagglutinin gene in a sample of H5N1 and then mixed it with genes from the H1N1 flu variant (the virus behind the 2009 swine flu pandemic). The hemagglutinin gene is responsible for causing the virus to create the proteins that allow for bonding to host cells. The purpose of the experiment was to create a virus that would be both deadly and highly contagious in humans, so that a means could be developed to defeat such a threat before it arose in the natural world.
In this new effort, the research team found that the proteins produced by the mutated version of the H5N1 held a 200-fold preference to bind with human cells over avian cells. This suggests, the researchers claim, that the mutated virus is much more like the viruses that caused earlier pandemics, and is thus much more of a threat to humans than the natural variant. The inference is that if H5N1 mixed (by having a single host become infected with both viruses at once) with other influenza viruses naturally, than there exists the possibility of a natural version of the mutant virus coming to exist and a very real threat to the health of humans across the globe.