In 2009, swine flu created a minor pandemic, only killing about 15,000 people but infecting millions more. The avian flu H9N2 is ravaging bird populations throughout Asia. They're bad enough on their own...but what if they joined forces?

The swine flu pandemic of 2009 was one of the worst flu scares in recent memory, even if its actual effects ended up being relatively moderate. Now something unambiguously good could come of all this: a universal flu vaccine.

To be more specific, people's immune response killed them. The virus was especially deadly to young and middle-aged adults because their immune systems were primed to kick into a fatal, antiviral overdrive.

Scientists now have an idea of just how the H1N1 virus may be so deadly, and what makes it different from earlier viruses from the same family. Hint: It's where it goes inside your body that counts.