The new flu which has emerged in China has unique traits, say scientists.
It is able to infect both the nose, giving it the potential to spread easily, and penetrate deep in the lungs where it causes pneumonia.
The authors of the American Journal of Pathology say the twin attack has not been detected in previous bird flus.
Meanwhile, a separate study has taken the early steps towards a vaccine for another emerging virus, Mers-coronavirus.
There have been 135 people infected with avian influenza A(H7N9) and 44 deaths since the outbreak started in Spring.
However, restrictions on live poultry markets have largely curbed the number of infections.
The study, by the Erasmus University Medical Centre in The Netherlands, looked at what parts of the body the virus could bind to and infect.
Infections like the common cold spread easily as they infect the upper respiratory tract, the nose and throat, so sneezing releases a lot of viruses into the air.
Other more deadly infections, such as the H5N1 bird flu, infect the lower respiratory tract deep in the lung where they can cause deadly pneumonia.
One of the researchers Prof Thijs Kuiken told the BBC the new bird flu could do both: “This has not been shown for this virus before.
“The study points to the fact that the virus has the potential to transmit easily in people and give pneumonia.”
A separate infection called Mers-coronavirus, which is centred on Saudi Arabia, has infected 114 people and killed 54.
Researchers at the University of Madrid have created a mutated version of the virus, which could be the first steps towards creating a vaccine.
Their study in the journal, mBio, shows the modified virus can infect cells but struggles to spread round the body.
Prof Ian Jones, a virologist at the University of Reading, said: “These papers address the challenging question of what we can do about potentially emerging viruses.
“The Mers results produce a candidate vaccine that, while not currently required, could be a future therapy while the H7 influenza paper demonstrates the ability of the recent China strains to infect man, essentially a heads up for what to look for when assessing risk.
“They neatly cover both surveillance and prevention approaches to being one-step ahead of the virus.”