Why do people live so much longer today then they did in earlier times?

I just read an interesting discussion on whether a person should go see a doctor when sick, or instead daven and ask Hashem to cure him. The discussion revolved around the opinion of the Ramban who seems to say in his commentary on Chumash that someone who has real faith should not go to doctors but rather rely on Hashem to cure him. In any case, even those who allow/promote going to doctors make it very clear that the person should feel that the cure is from Hashem and not really from the doctors.

This raises a very big question. The average life expectency in countries like the US and Israel today is around 80 years. In Classical Rome which was around the time of Chazal the average life expectency was 20-30 and if a child survived childhood his life expectency his life expectancy was 47.5. How can we explain the dramatic increase in life expectency, after all life and death are in the hands of Hashem? If all cures are from Hashem then how come in the last 100 years the efficacy of cures has gone up tremendously? It is very hard to say that our generation is more worthy as this is against the concept of ירידת הדורות, which postulates that previous generations were greater then ours (this is a subject for a separate post) and that we can't even understand how great the Rishonim were let alone the Tannaim and Amoraim. Why would Hashem want our lowly generation to live so much longer then the generations of Chazal who were so much greater?