I remember reading something about the plumbing (which was made of lead back then) slowly inducing saturnism. This was validated to a degree by someone making a dish following an roman recipe and finding it overly salty, and a reduction of the taste acuity is linked with lead poisoning.
Since lead would also affect the intellect, it could have lead (no pun intended) to the demise of the roman empire.
Of course, the same loss in intellectual faculty could be a factor in the establishment of a silly religion.
