Through Adam's Sin His Whole Posterity Were Corrupted, and Were Born Under the Penalty of Death, Which He Had Incurred
Thence, after his sin, he was driven into exile, and by his sin the whole race of which he was the root was corrupted in him, and thereby subjected to the penalty of death. And so it happens that all descended from him, and from the woman who had led him into sin, and was condemned at the same time with him—being the offspring of carnal lust on which the same punishment of disobedience was visited—were tainted with the original sin, and were by it drawn through various errors and sufferings into that last and endless punishment which they suffer in common with the fallen angels, their corrupters and masters, and the partakers of their doom. And thus “by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned.” By “the world” the apostle, of course, means in this place the whole human race.
Source: The Enchiridion (New Advent)