ETA: I found this:
"Swine flu has killed at least 20 people in Mexico, and possibly more than 80. Most of the dead were ages 25 to 45, a worrying sign because a hallmark of past flu pandemics has been high fatalities among healthy young adults."
Really? What's up with that??
I just noticed your ETA. The answer this time is still "no one knows." In the 1918 pandemic, it's speculated that young and healthy adults had overachieving immune systems. It wasn't the virus itself that killed them, but the build up of inflammatory cells and substances in the lungs as the immune system responded to the virus. That's supported by some of the 1918 victims dying just a few hours after they first showed symptoms. The very old and very young did not have such vigorous immune systems, so they didn't get as sick. There's also speculation that the dead victims were more likely to have TB than the survivors, something supported by the sharp drop in TB rates in 1919. The pandemics of 1957 and 1968 had the highest death rates in the very young and the elderly. All pandemics share the trait that the virus is new and novel to humans, so the population doesn't have any immunity built up to it. That feature, and the apparent person-to-person transmission pattern are what make this current outbreak so concerning.
