Objection 1. It would seem that the aforesaid powers are not more infected than the others. For the infection of original sin seems to pertain more to that part of the soul which can be first the subject of sin. Now this is the rational part, and chiefly the will. Therefore that power is most infected by original sin.
Objection 2. Further, no power of the soul is infected by guilt, except in so far as it can obey reason. Now the generative power cannot obey reason, as stated in Ethic. i, 13. Therefore the generative power is not the most infected by original sin.
Objection 3. Further, of all the senses the sight is the most spiritual and the nearest to reason, in so far "as it shows us how a number of things differ" (Metaph. i). But the infection of guilt is first of all in the reason. Therefore the sight is more infected than touch.
On the contrary, Augustine says (De Civ. Dei xiv, 16, seqq., 24) that the infection of original sin is most apparent in the movements of the members of generation, which are not subject to reason. Now those members serve the generative power in the mingling of sexes, wherein there is the delectation of touch, which is the most powerful incentive to concupiscence. Therefore the infection of original sin regards these three chiefly, viz. the generative power, the concupiscible faculty and the sense of touch.
I answer that, Those corruptions especially are said to be infectious, which are of such a nature as to be transmitted from one subject to another: hence contagious diseases, such as leprosy and murrain and the like, are said to be infectious. Now the corruption of original sin is transmitted by the act of generation, as stated above (Question 81, Article 1). Therefore the powers which concur in this act, are chiefly said to be infected. Now this act serves the generative power, in as much as it is directed to generation; and it includes delectation of the touch, which is the most powerful object of the concupiscible faculty. Consequently, while all the parts of the soul are said to be corrupted by original sin, these three are said specially to be corrupted and infected.
Reply to Objection 1. Original sin, in so far as it inclines to actual sins, belongs chiefly to the will, as stated above (Article 3). But in so far as it is transmitted to the offspring, it belongs to the aforesaid powers proximately, and to the will, remotely.
Reply to Objection 2. The infection of actual sin belongs only to the powers which are moved by the will of the sinner. But the infection of original sin is not derived from the will of the contractor, but through his natural origin, which is effected by the generative power. Hence it is this power that is infected by original sin.
Reply to Objection 3. Sight is not related to the act of generation except in respect of remote disposition, in so far as the concupiscible species is seen through the sight. But the delectation is completed in the touch. Wherefore the aforesaid infection is ascribed to the touch rather than to the sight.
Source: Summa Theologica (New Advent)