Why has life expectancy increased around the world?

The only reason I could think of is advancments in the medical field and medicine. America for example, one of the worst diets in the world, this country is killing itself, its sickening, but how is the life expectancy up there. It must be that much more illneses are treated or helped through the use of medicine. In parts of rural russia, like the caucasus mountains people live well to 90 or 100, that is if nothing happens.