Answer:
The word repeated throughout the Prologue of Oedipus Rex is "pollution" or "polluted."
Textual evidence:
"Our city, look on this, the root of all our evils, / pollution unfathomable" (lines 21-22)
"Your suppliant at your hearth, hear me, Oedipus, / who all men call the Great" (lines 29-30)
"With that pollution on your head, no prayer / can call down any god to be your friend" (lines 35-36)
"The country reeks with the pollution of our rotting / corpses dying, unburied" (lines 77-78)
"If you could see yourself, I think you would not say / you felt no pang, no stirrings of the heart" (lines 98-99)