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 and do not infect humans - this sort is known as low pathogenic avian influenza (LPAI).

However, some types of bird flu are more harmful for certain birds and animals. They are called highly pathogenic avian influenza (HPAI). Since 1997, a particularly aggressive strain of highly pathogenic bird flu known as A/H5N1 has emerged. It was first identified in Hong Kong where influenza cases are well reported but may have been around for longer than that elsewhere in the Far East. The A/H5N1 strain of bird flu seems to kill a high proportion of birds infected. It also infects a surprisingly wide range of birds and animals.