Just mother nature taking another whack at the two legged pest. And remember, this may be a variant of the original version of the virus. They are always adapting. Generally they finally adapt toward less virulence over time because if, as a virus, you are too effective at killing hosts too quickly, then you don't survive so evolutionary forces (if you believe in that kind of thing...) tend to favor less virulent versions over time. There are many things waiting in the wings for an opportunity to get at the two legged pest. There are likely some we don't even know about yet buried in forests we keep cutting down.
People tend to not support Public Health over time often because they do not understand it or what it does. AND, when public health succeeds, nothing happens so it is constantly forced to prove the negative regarding its value. We literally have had county level governments dissolve their health departments because they didn't see the value....and then have to reactivate them when a nasty bug (pertussis) started running through the school system. Public health interventions and a diminished reservoir likely solved the problem after about a year. Oh and public health tends to be run by Epidemiologists and they are horrible at marketing themselves or the value of public health.
Public health should be supported as much as the military in my opinion (biased of course), but most people have no clue that it is a very thin line of surprisingly highly trained people standing between you and disease impacts until they hit the public and then the public wants someone to blame and public health seems like a perfect scapegoat.
If you noticed, about once every three to five years a new one rears its head on the populace lately. The first big one in modern times has been HIV. We, as a world, really dodged a bullet on H1N1. For a few weeks in the beginning I was thinking as many as a billion people could go down but fortunately it was less virulent than feared but just a few genomic adaptations in many flu viruses could end up with that result. Oh, and something people don't realize, but the relatively mild 1957 flu pandemic was likely a lab released version from Russia, so humans could do it to themselves helping mother nature out. We can now create our own viruses and custom make them. There are fanatics out there the just might do it because of their particular belief system. We are so lucky.
Public health is right now desperately trying to develop a universal flu vaccine. May be close, Maybe not.
Oh and thank you to all the anti-vaxers out there. You keep me well employed... and it is never a dull job. Most outbreaks have an index case that is not vaccinated. I just hope we can all hold it together till I die of natural causes... So I can Camp!!!!