Whether it's flu or not, a virus is a (the?) big pandemic fear in the short term. Or An MDR bacterium (staph, anyone?)
But flu is a good candidate for a lot of reasons: rapid mutation, limited vaccine utility if we guess wrong about the dominant strain (note: I get a vaccine annually, but that doesn't mean I don't recognize the limitations), airborne, lives a decent time outside the body, takes time to manifest symptoms- plenty of time to catch a flight. All we need is a particularly virulent strain. If it's also particularly infectious...
I was very much exposed to family memories of 1918, as was my husband in his family. A walk through the cemetery is a pitiful thing if you start reading off dates from around then. A new flu would have to work differently, and far fewer would die of dehydration and secondary infections, but it's could happen.
But there are also other candidates out there. My thought is to be realistic - aware but not worrying constantly. There's no point in doing more than that.