Fourteen years ago I worked at a desk opposite a Civil Servant who was contingency planning for Diplomatic Missions for a pandemic flu outbreak when it happens. We didn't have the most cheery conversations.
There is a school of thought that 'Spanish' originated in East Asia - and back in the day, of course, global travel was both time bound and rare.
Off the top my head (I'm not a Virologist by education, so don't rely on the figures), seasonal flu has around an 80% contagion rate, but a relatively small mortality rate of 20% - of which are elderly, or have a weakened immune system through a pre-existing condition.
H5N1 has a mortality rate of 60%, but isn't very good a going cross species, you need to be in close proximity to birds. It has, however, gone human to human.
When H5 mutates, and does become more effective at human to human contagion, coupled with our ability for, and ready access to, international travel, things will become 'interesting'.
It was described to me, in the UK context, as 'walk down the street that you live, and every third house, say 'you're all dead'.