For some reason, life expectancy plummeted with the fall of the Roman Empire, and it did, indeed, reach very low levels and didn't really climb back to "normal" until the industrial revolution. Was it hygiene? The foods eaten?
Warfare? The Plagues? Probably all of the above.
Add lead chronic intoxication for Romans (lead was used excessively in their aquaducts) and lots of criminality in the dark ages (sea piracy and land armed thieves) later in the Middle ages. And underdeveloped obstetrics. Also, civilian life. Too many people living in crowded conditions, drinking from the same public taps etc.
This is among the first things taught in the medical school here. Hygiene (and prevention in general) is much better and cheaper for the community and the individual.
Sorry for off topic, but a comment about prevention can't hurt.