http://www.cnn.com/2009/HEALTH/04/27/gupta.qanda/index.html
CNN: Sanjay, one question that we haven't gotten to ... most of the people who died from swine flu in Mexico were in the prime of their lives really, and this usually hits infants or the elderly. What does that say to you as a doctor?
Gupta: This is interesting. And the same thing happened in sort of a nonintuitive way when we were talking about SARS and when we were talking about avian flu.
Think about it like this: Typically, you think of someone who has a weakened immune system, who's going to be most adversely affected by an infection. Their immune system simply can't fight it.
But in these cases, it's the immune system itself that reacts robustly, and it's the immune system in that reaction to the virus that is causing death in these patients. So the virus starts that cascade, but all that fluid builds up in the lungs, and all those inflammatory cells throughout the body -- that's what's causing the problem. We saw the same thing with SARS and with avian flu as well.
Which is why exactly as you said ... [people in their] 20s and 30s and 40s, this hospital behind me, they say that's been the bulk of their patients with regard to swine flu.