] During replication, when the virus invades and colonises
] human cells, the RNA makes errors, resulting in genetic
] mutations to the prongs. Such mutations are known as
] 'antigenic drift'. The reason they are dangerous is that
] they trigger small changes to the haemagglutinin,
] enabling the prongs to stick better to human cells and
] evade the antibodies produced by the immune system. In
] addition, type A viruses such as H5N1 can also 'swap' or
] re-assort genetic material with other viruses. This
] process, known as 'antigenic shift', occurs when animal
] strains mix with human strains, producing viruses to
] which no animal or human has immunity.
]
] 'The problem is, one chicken can contain hundreds of
] thousands of strains of H5N1,' explains Oxford.

A long, scary, interesting article about Bird Flu.