10. Influenza

Influenza, the flu, is a viral infection that many people suffer seasonally. What you may not realize is that the flu is a serious illness, and it can pose life-threatening complications and death for some people. Every year, an estimated three to five million people around the world suffer from a severe form of the flu. Worse, 250,000 to 500,000 of them will die -- and those are just the estimates for industrialized countries [source: WHO].

While everyone should consider getting an annual flu vaccine, it's recommended that the elderly and the very young be vaccinated every year. This applies to people 50 or older -- especially those who live in nursing homes -- and kids between 6 months and 2 years old . Anyone with a chronic medical condition or weakened immune system should be vaccinated, as well.

If you're not one of the millions already getting your annual flu shot, this might make you reconsider: The influenza vaccine prevents an estimated 70 to 90 percent of flu infections.