There are many theories as to why the Roman Empire fell. One of the theories is lead poisoning. An ancient Roman writer named Vitruvius indicated in his book, De Architectura that the Romans knew of the danger of lead pipes and, consequently, Terra Cotta was preferred. I do not think that this was a reason of the fall of the Roman Empire because less than half of the population had pipes. Most Ancient Romans used wells that were on their land.
The people that did not have wells used lead pipes because lead was abundant and easily malleable.
Another theory is that Christianity weakened Roman Virtues. Many writers argued that Christianity attracted the least intellectual and most superstitious people in the Empire. That not surprisingly did the triumph of Christianity in the Empire helped the downfall of Rome. A man named Edward Gibbon suggested that "Christians threatened with their ideas of forgiveness and mercy, the severe patriotic virtues of the Romans that enabled the to resist Heroically the Barbarian invasions.