On November 13, Spain’s Ministry of Agriculture, Fisheries and Food imposed the confinement of all free-range poultry farms to curb the spread of avian influenza – more commonly known as “bird flu” – caused by the H5N1 virus. This drastic, though not unprecedented, measure was justified by climbing numbers of outbreaks in Europe, as well as the heightened risk posed by the southward migration of wild birds in Europe.

This type of news understandably causes alarm among the general public, but has the risk of the H5N1 virus triggering a pandemic among humans actually increased? And why is it causing increasing devastation not only among birds, but also among many species of mammals?

Flu viruses come in many forms. There are four strains (A, B, C, and D), each with many different sub-types.

Type A viruses are the most significant for human and animal health, and they are all descended from avian viruses. The flu A virus has two essential proteins in its envelope: haemagglutinin (H) and neuraminidase (N). There are 18 types of H and 11 types of N, which can be found in any combination: H1N1, H1N2, and so on up to H18N11.

There is also considerable genetic diversity within each type. This means strains belonging to the same type may differ in their ability to cause disease, their range of possible hosts, transmissibility, and so on.

The H5N1 virus is a type of highly pathogenic influenza A virus with a high mortality rate among birds. Wild species, especially those in aquatic environments, are the virus’ natural reservoir.

H5N1 was first detected in domestic geese in China in 1997, and it has since spread widely across several continents via migratory birds, diversifying into different genetic groups or clades. The virus has spread to a large number of bird and mammal species across the globe, and is now the cause of a very real pandemic in........

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