Influenza viruses undergo random mutation all the time as do Corona viruses like SARS and all the other organisms that infect us. Some do mutate more readily, some replicate more abundantly. But a genetic change that make a virus or bacteria capable of causing a pandemic could come from many currently circulating organisms.
HIV emerged when a road was built across Uganda, literally. Truck drivers and prostitutes along the road allowed a virus that had been confined to a small geographic area to break out. From there a variety of just right circumstances allowed the disease to become a world wide pandemic. And it is indeed a major pandemic. Since it kills so slowly, the effect on the world's psyche is quite different than if it were a flu pandemic which kills within days.
The black death (plague) of the middle ages became a pandemic when rats on ships brought it to cities that were crowded and infested with rats and fleas. Tuberculosis became a pandemic when rural dwellers moved into crowded cities.
Sometimes it is the organism that changes, sometimes it is other circumstances that allow a disease to spread. Rheumatic fever from strep throat infections, rapidly invasive group A strep, meningococcal disease, and many other bacteria can go from mild or no disease to rapidly fatal by just having small genetic variations. Corona viruses can cause a mild cold or fatal SARS with very small genetic differences.
Just about any infectious organisms can mutate into a deadly version by mere chance. After that, milder versions tend to be selected for as the worst cases tend to kill the host before spreading.
The reason everyone is so worried about a pandemic of influenza is because they have occurred on a regular basis in the past. In addition, the influenza pandemics of the past have been preceded by the development of a substantially new virus in birds. It's like earthquakes, we know they will happen we just don't know when.
There are some researchers who think H5 viruses will never become highly contagious in humans. On the other hand, this particular strain is following the pattern of past pandemics. What is really happening is we know a lot about human flu pandemics but we don't know quite enough to know if this is the beginning of one or not.
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