Influenza - or, as it is often known, flu - is a large family of different viruses (more than 70 major types), some of which affect humans and many of which affect other animals and especially birds.
Flu in birds is quite common. Indeed birds are thought to be the natural hosts for these infections with wild fowl (wild ducks etc) playing the most important role. Most strains of bird flu (or avian influenza to give it its scientific name) are relatively harmless to their natural bird hosts, do not infect humans often and only cause mild disease when they do.
However each novel influenza has to be evaluated for its potential to cause severe disease, a pandemic or both.
Some types of bird flu are more harmful for certain birds and animals. Since 1996, a particular strain of bird flu known as A(H5N1) has emerged. First identified in Southern China and Hong Kong, it may have been around for longer than that elsewhere in the Far East. The A(H5N1) viruses kill a high proportion of poultry that they infect. They also infect a surprisingly wide range of birds and animals and have persisted over time and spread to poultry in a number of countries with poor infection control in poultry flocks.
A(H5N1) occasionally infects humans in contact with infected poultry. Because it causes severe disease in humans, it has to be taken seriously by European public health and animal health authorities. Though occasionally present in European wild birds, A(H5N1) viruses have never become established in poultry in Europe because of high levels of biosafety. Also the viruses have not adapted to be able to infect humans easily or transmit from person to person with any efficiency.
In March 2013, avian influenza A(H7N9) virus has emerged in China, causing severe disease in humans.