I believe that part of the reason why the 1918 flu had such a predilection for the relatively young adults was actually because their immune response WAS strong. Patients drowned in their own fluids from the secretions that built up in their lungs. Googling around on "cytokine storm" and influenza will bring up some good information. At any rate, with some viruses, including avian flu to the best of my knowledge, it may not be the virus per se that is the problem, but rather the robust immune response (cytokine storm) that causes the problems.

Eric