1) Reduction in infant mortality;
2) Antibiotics and other treatments for infections/diseases;
However, if you were to look at the biggest causes of mortality it's...
1) Obesity
2) Cancer
Both of these things are linked to an overabundance of food / sweets and a lack of exercise.
While it's great that life expectancy overall is increasing, I don't think that this necessarily means that those who are living longer are living healthier and more capable lives (on average).