[Oorang]

What I don’t understand is why healthier people’s immune systems overreact and people over 50 do not, for example.

It is due to the cytokine storm. Which is explained here:
http://www.fluwikie.com/index.php?n=Science.PrimerCytokineStorm

Or here:
http://www.wisegeek.com/what-is-the-cytokine-storm.htm

Or there is more out there on the www.

[Paved Paradise]

Thanks. I understand it somewhat but I wasn’t sure.