Today, Most Deaths Caused by Lifetime of Action or Inaction
Our instincts lead us to fear the unknown and immediate threats (probably so we can be ready to run – or maybe fight). But today the biggest risks to an untimely dealt are not lions, other people out to get us, or even just random infection. We have to adapt to the new risks by taking action to eat healthfully and exercise, in the same way we we have evolved to avoid becoming a meal for a hungry beast.
Today the largest causes of death are heart disease and cancer (which account for more than 60% of the deaths causes by the top 10 leading causes of death). The next leading causes are non-infectious airways diseases, cerebrovascular diseases and accidents. Alzheimer’s, diabetes, nephropathies, pneumonia or influenza and suicide make of the rest of the top 10 leading causes.
In 1900 Pneumonia or influenza and tuberculosis took as many lives (per 100,000 people) and cancer and heart disease take today. We have done well decreasing the incidents of death (fewer deaths per 100,000) by greatly reducing and nearly eliminating some causes of death (the 2 leading causes from 1900 are good examples).
Heart disease has grown to take even more people today than it took in 1900 which is a sign of two things. First our failure to live heart healthy lives (largely about exercise and over eating). And secondly, as we eliminate some causes of death people survive those, to then be taken by another cause (heart disease and cancer are both increased by this).
Seeing how many deaths were caused by tuberculosis, can make you appreciate why dangerous drug-resistant strains of TB are such a concern . The risks of death caused by the improper use of anti-biotics  are real and worrisome.
Chart from New England Journal of Medicine interactive application with article on The Burden of Disease and the Changing Task of Medicine 
The article also includes some interesting discussion on the challenges and public policy issues around collecting data on human health.
Related: Leading Causes of Death (2007)  – Study Finds Obesity as Teen as Deadly as Smoking  – Cancer Deaths, Declining Trend?: I doubt it  – Cancer Deaths Increasing, Death Rate Decreasing 
We are not going to eliminate heart disease or cancer in the next 10 years. But a significant number of those deaths are due to bad decisions for decades – smoking, eating poorly, not exercising enough, etc.. Healthy living won’t guaranty you a long healthy life but it will give you much better odds for a longer and healthier life. Our bodies are able to cope with an awful lot of abuse, but we can help a great deal by maintaining healthy habit which give our bodies more strength to fight off problems. For example, exercise doesn’t just keep our weight down, it exercises the heart and it can reduce the impact of mental stress (at least this is my impression – I can’t remember if I have read studies that make this claim fairly certain).