A - Great question. Thanks for asking. The simple answer is that just as humans share in the redemption of Christ, so we also share in the fall with Adam and Eve.
Just as through one person sin entered the world, and through sin, death, and thus death came to all, inasmuch as all sinned - for up to the time of the law, sin was in the world, though sin is not accounted when there is no law. But death reigned from Adam to Moses, even over those who did not sin after the pattern of the trespass of Adam, who is the type of the one who was to come. But the gift is not like the transgression. For if by that one person's transgression the many died, how much more did the grace of God and the gracious gift of the one person Jesus Christ overflow for the many. And the gift is not like the result of the one person's sinning. For after one sin there was the judgment that brought condemnation; but the gift, after many transgressions, brought acquittal. For if, by the transgression of one person, death came to reign through that one, how much more will those who receive the abundance of grace and of the gift of justification come to reign in life through the one person Jesus Christ. - Romans 5:12-17We all share a nature and when Adam was tested, it wasn't so much as an individual, but as the representative of all of humanity. Thus, all of humanity failed the test and fell from grace.
Thus, the Catechism teaches:
404 - How did the sin of Adam become the sin of all his descendants? The whole human race is in Adam "as one body of one man". By this "unity of the human race" all men are implicated in Adam's sin, as all are implicated in Christ's justice. Still, the transmission of original sin is a mystery that we cannot fully understand. But we do know by Revelation that Adam had received original holiness and justice not for himself alone, but for all human nature. By yielding to the tempter, Adam and Eve committed a personal sin, but this sin affected the human nature that they would then transmit in a fallen state. It is a sin which will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of original holiness and justice. And that is why original sin is called "sin" only in an analogical sense: it is a sin "contracted" and not "committed" - a state and not an act.