Swine flu

This swine flu issue seems already to be getting out of hand -- I mean, only 20 cases in the US so far? No deaths? Let's not forget that the regular flu kills, on average, about 100 people in the US per day.

One interesting thing, though, is that, like other flus, this strain seems to disproportionally affect young adults. Here's a graph of the Spanish flu from Wikipedia:



Young adults (ages from about 18 to 25) are in the prime of their lives -- why are they more susceptible than those younger and older than them? Is it that they're more social than those groups?

(And why are they more social? I've found this in my own life, even though I don't really understand it. When I was younger I used to run around all over the place doing all kinds of things. Now I don't. What's changed?)