Here's part of the section on evolution of new strains of flu. Avian flu is an example of Type A influenza. Type A has the potential to be so very deadly because it can mutate in a host through an astonishing process called genetic reassortment.
If a cell is simultaneously infected by two different strains of type A influenza, the offspring virions may contain mixtures of each parents' genes!
These newly created mixed genomes are very different from their parents and (probably) have never been "seen" by your immune system - or for that matter, anyone else's. This form of viral evolution is called antigenic shift, to differentiate it from antigenic drift (which occurs slowly and without a change in the gene associations). These new combinations present us with such a unique strain of virus that our immune system has to start all over to make new antibodies to combat it.
As if that weren't amazing enough, influenza A can infect other mammals (other than humans) and even birds! It's VERY unusual for a virus to have such a wide host range, but influenza A somehow manages this trick. It probably has to do with the fact that the virus gains entry using receptors common to many species. That means a strain of influenza A may worry one species for decades and then suddenly jump to a new species! This sudden jump, due to antigenic shift, can produce a very serious epidemic. For example, about a decade ago many seals washed up on the eastern seaboard of the USA dying from a strain of influenza A that, until then, had only been found in birds! Horse and swine influenza A have turned up in humans. Influenza A is the nightmare of science fiction - a virus that normally causes only a slight illness, undergoes genetic recombination with other species and comes back as a killer virus! Fact is, influenza A has been conducting random, unlicensed recombinant genetics "experiments" for centuries and will continue to do so regardless of our feelings on the subject.