[Big_Jules]

It might be worth adding to Chris' discussion of influenza (the virus responsible for the 'true flu') that the virus can mutate in each of two major surface proteins, the haemagglutinin (H) and the neuraminidase (N), both of which are important in general for infection. This is where such descriptions as H5N1 come from. Either of these two molecules can change in subtle ways, called antigenic drfit, or in more dramatic ways, called antigenic shift. These refer to changes that are relevant as to how our bodily defences, specifically the immune system, 'see' the virus. Major outbreaks, including pandemics (global spread) are usually associated with strains that have undergone antigenic shift. These are unknown by our defences and attack readily. Viruses that have undergone drift may be sufficiently similar to be recognised by immune memory, including the immunity conferred by immunisation.

It may also be, as with other pathogenic organisms that some changes may also increase the virulence, or the ability of the agent to cause disease.