Objection 1. It would seem that death and other bodily defects are not the result of sin. Because equal causes have equal effects. Now these defects are not equal in all, but abound in some more than in others, whereas original sin, from which especially these defects seem to result, is equal in all, as stated above (Question 82, Article 4). Therefore death and suchlike defects are not the result of sin.
Objection 2. Further, if the cause is removed, the effect is removed. But these defects are not removed, when all sin is removed by Baptism or Penance. Therefore they are not the effect of sin.
Objection 3. Further, actual sin has more of the character of guilt than original sin has. But actual sin does not change the nature of the body by subjecting it to some defect. Much less, therefore, does original sin. Therefore death and other bodily defects are not the result of sin.
On the contrary, The Apostle says (Romans 5:12), "By one man sin entered into this world, and by sin death."
I answer that, One thing causes another in two ways: first, by reason of itself; secondly, accidentally. By reason of itself, one thing is the cause of another, if it produces its effect by reason of the power of its nature or form, the result being that the effect is directly intended by the cause. Consequently, as death and such like defects are beside the intention of the sinner, it is evident that sin is not, of itself, the cause of these defects. Accidentally, one thing is the cause of another if it causes it by removing an obstacle: thus it is stated in Phys. viii, text. 32, that "by displacing a pillar a man moves accidentally the stone resting thereon." In this way the sin of our first parent is the cause of death and all such like defects in human nature, in so far as by the sin of our first parent original justice was taken away, whereby not only were the lower powers of the soul held together under the control of reason, without any disorder whatever, but also the whole body was held together in subjection to the soul, without any defect, as stated in the I, 97, 1. Wherefore, original justice being forfeited through the sin of our first parent; just as human nature was stricken in the soul by the disorder among the powers, as stated above (3; 82, 3), so also it became subject to corruption, by reason of disorder in the body.
Now the withdrawal of original justice has the character of punishment, even as the withdrawal of grace has. Consequently, death and all consequent bodily defects are punishments of original sin. And although the defects are not intended by the sinner, nevertheless they are ordered according to the justice of God Who inflicts them as punishments.
Reply to Objection 1. Causes that produce their effects of themselves, if equal, produce equal effects: for if such causes be increased or diminished, the effect is increased or diminished. But equal causes of an obstacle being removed, do not point to equal effects. For supposing a man employs equal force in displacing two columns, it does not follow that the movements of the stones resting on them will be equal; but that one will move with greater velocity, which has the greater weight according to the property of its nature, to which it is left when the obstacle to its falling is removed. Accordingly, when original justice is removed, the nature of the human body is left to itself, so that according to diverse natural temperaments, some men's bodies are subject to more defects, some to fewer, although original sin is equal in all.
Reply to Objection 2. Both original and actual sin are removed by the same cause that removes these defects, according to the Apostle (Romans 8:11): "He... shall quicken... your mortal bodies, because of His Spirit that dwelleth in you": but each is done according to the order of Divine wisdom, at a fitting time. Because it is right that we should first of all be conformed to Christ's sufferings, before attaining to the immortality and impassibility of glory, which was begun in Him, and by Him acquired for us. Hence it behooves that our bodies should remain, for a time, subject to suffering, in order that we may merit the impassibility of glory, in conformity with Christ.
Reply to Objection 3. Two things may be considered in actual sin, the substance of the act, and the aspect of fault. As regards the substance of the act, actual sin can cause a bodily defect: thus some sicken and die through eating too much. But as regards the fault, it deprives us of grace which is given to us that we may regulate the acts of the soul, but not that we may ward off defects of the body, as original justice did. Wherefore actual sin does not cause those defects, as original sin does.
Source: Summa Theologica (New Advent)