Thomas Aquinas On Original Sin, Part 1.

From Summa Theologica:

Treatise on Habits in Particular, Question 81, Article 1: Whether the first sin of our first parent is contracted by his descendants, by way of origin?

Thomas lists four philosophical objections to the transmission of Original Sin, but only one possible biblical objection- that from Ezekiel 18:20, "The son shall not bear the iniquity of the father." Thomas then replies from Romans 5:12, "By one man sin entered into this world, and by sin death." Thomas points out that a denial of the transmission of Original Sin is part of the Pelagian heresy.

Thomas directly replies to the objection raised from Ezekiel 18:20 with the following:

The son is said not to bear the iniquity of his father, because he is not punished for his father's sin, unless he share in his guilt. It is thus in the case before us: because guilt is transmitted by the way of origin from father to son, even as actual sin is transmitted through being imitated.