Pandemic Flu-What is it?

How Is Pandemic Flu Different from Seasonal Flu?
https://www.cdc.gov/flu/pandemic-resources/basics/about.html

Includes:

How often does it happen?
Will most people be immune?
Who is at risk for complications?
Where can I get medical care?
Will a vaccine be available?
Will antivirals be available?
How many people could get sick and suffer complications?
What impact will it have on schools and workplaces?