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Originally Posted by Ken G
Thanks beskeptical, it sounds like you have a great deal of knowledge about this and are good at communicating it clearly. The thing I still don't understand is, if a virus that already existed in birds but wasn't so deadly can mutate to one that is deadly for birds, then why do human pandemics have to start with birds? If a bird pandemic can appear in birds all by themselves, why would it not be easier to get human pandemics from humans all by themselves?
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Human pandemics can begin in humans. HIV is one such pandemic. Though the virus spread from other primates to humans a few 1,000 years ago, it was the change in circumstance that led to the current pandemic.
But there are reasons pandemics begin in other species. When a disease is particularly deadly, it kills the host too fast for the infection to spread. So milder disease is often selected for genetically since it spreads more easily. That makes deadly pandemics less likely to emerge in the species being killed.
In the case of bird flu, though domestic birds are dying at very high rates, many wild birds with the same infection are not, and neither are pigs which is also a problem. The virus then has the opportunity to evolve into something that spreads among one species while becoming very lethal to another. When events occur that allow the virus to jump to another species, the lethality can be very high. After the pandemic runs its course, milder organisms are selected which don't kill the host.
In other words, the mechanisms of natural selection have given us the recurring deadly bird flu pandemics. It just so happens human and bird genetics are a set up for this particular virus to evolve in birds and occasionally jump to humans. Domestic birds are a big part of the equation since close human bird contact occurs there.
By the way, we also get bird flu virus infections that are not deadly. They are now referred to as low pathogenic avian influenza viruses, LPAI. The current H5N1 is considered a high pathogenic avian influenza virus, HPAI.