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!
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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.
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