The Influenza virus changes constantly, mutating in small steps (called antigenic drift) or in great leaps (called antigenic shift). The virus is able to infect many species of animals, including humans. We fall victim to the virus according it its ability to infect the nose, our airways and our lungs.
Research has now revealed that Influenza H7N9, the strain that is scouring China at the moment, 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 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 Influenza H5N1 (or bird flu), infect the lower respiratory tract deep in the lung where they can cause deadly pneumonia.
 Richard et al: Limited airborne transmission of H7N9 influenza A virus between ferrets in Nature - 2013