Likewise, [St Augustine] notes that death reigned even over those
"who had not yet sinned of their own individual will, as Adam did, but had drawn from him original sin...because in him [Adam] was constituted the form of condemnation to his future progeny'' (
Ibid. I.13; Fathers of the Church).
St Thomas writes that,
"Wherefore, original justice being forfeited through the sin of our first parent; as human nature was stricken in the soul...also it became subject to corruption'' (
Ibid. IaIIae.85.5; New Advent). Through removal of original justice,
"the sin our our first parent is the cause of death and all such like defects in human nature'' (
Ibid.; New Advent).
St Thomas continues, distinguishing between Adam's willful 1st sin and the effects of Original Sin of the rest of the human race, who are not guilty of Original Sin through their bad will but through nature. ...Though original sin is a sin of the will of Adam, it is not a sin of his descendants "
except inasmuch as this person [his descendant] receives his nature from his first parent, for which reason it is called the `sin of nature'' (
Summa Theologica IaIIae.81.1; New Advent
First parent = Adam
Is not Christ, then, subject to Original Sin because He is an offspring of Adam?
Thomas Aquinas, on the other hand, finds that Jesus Christ, by virtue of his conception, would not be subject to original sin. Aquinas found that original sin passed to men since they were "
one body'' with Adam (
Summa Theologica IaIIae.81.1; New Advent). But Christ was not part of this body. As Aquinas notes, original sin is only contracted by those
"who are descended from him [Adam] through seminal power'' (
Ibid. IaIIae81.4; New Advent).
In other words, only those properly of the seed of Adam would be subject of original sin. Thus Aquinas concludes that ,
''If anyone were to be formed by God out of human flesh, it is evident that the active power would not be derived from Adam. Consequently, he would not contract original sin'' (
Ibid.; New Advent). Thus, Aquinas finds that Jesus Christ would have no need to cleanse his body of original sin, since his conception, by its independence from carnal generation, would have been without sin.
http://www.memoryhole.net/~chris/research/original_sin.html