CNN:
Researchers do not know how the virus is jumping relatively easily from person to person, or why it’s affecting what should be society’s healthiest demographic. Many of the victims who have died in Mexico have been young and otherwise healthy.
It’s too early to say if this is the mechanism (or even if a genuine trend exists making young people more susceptible), but one possible explanation is a cytokine storm, in which the body’s immune system reacts so violently that it causes damage. There’s an interesting discussion of all this over at ScienceBlogs. Luckily for me, my immune system kinda sucks.