I think we're living longer than ever, average lifespan in the us is 78.4 years in the world 68.9! Considering 1900's it was 48.3...that's some impressive changes in the world!
Life expectancy graphs here: http://www.google.com/publicdata?ds=wb-wdi&met=sp_dyn_le00_in&tdim=true&dl=en&hl=en&q=average+lifespan+world#met=sp_dyn_le00_in&tdim=true
and article with chart here: http://www.ajcn.org/content/55/6/1196S.full.pdf
In regards to something like cancer, it seems that we hardly even identified it until the 1930's...but that's more because we didn't understand it in the way we do now than that it's become 'more prevalent'. http://cancer.about.com/od/historyofcancer/a/cancerhistory.htm
It seems the average age of cancer, in general, is in the 60's and 70's: http://seer.cancer.gov/csr/1975_2003/results_single/sect_01_table.11_2pgs.pdf
I guess my thought is since we're living so much longer than we used to don't we really just see more of the cumulative diseases taking us out instead of the communicatable bacterial one's? People who had poor immune systems an/ord are easily susceptible to disease (genetic/lifestyle diseases) would be wiped out when the were young without all these advances.
All this isn't to say that we can't live more healthy as a society (US particularly). Obesity is as high as 34.4% in some states! (http://www.cdc.gov/obesity/data/trends.html
) and Diabetes (estimates at 8% of the population now) are rampant. Most of this is caused not my 'food science' like Genetically Modified crops but by the food products like Twinkies, Jolt Cola, etc...which is really a learned behavior/choice.
I don't think all that bad stuff is all a result of the 'food science' but more in how we make our food. I don't see how we can feed the populations of the world without crop resistant seeds. I don't think there's anything wrong with splicing salmon genes into tomato's for cold resistance...problems can probably definitely arise, but we have to become more efficient in feeding the population somehow.