Think left and think right and think low and think high. Oh, the things you can think up if only you try!
~ Dr. Seuss (March 02, 1904 - September 24, 1991)
In normal flu years, most of us have immunity, either from the vaccine or from having the flu in previous years. But in pandemics, we have no prior immunity, and it's just like being hit with a completely new disease that we've never built up any ability to fight. That's why the mortality tends to be high even in the age groups that don't usually get very sick from flu.