I'm surprised that nobody has mentioned cyclists being killed in accidents yet!
I did see some figures a while back that took accidents into account and cyclists still gained significantly on average. I.e the health risks of a sedentary lifestyle greatly outweighed the risk of being killed or crippled in an accident while out on your bike.
The generally accepted figures are the health benefits of cycling extend your life on average by 2 years. The risks of cycling shorten it by one week. So that's worth an extra half hour a day every day of your life. Seems like a good trade to me.