It seems that original sin is not habitude. For original sin is lack of original justice, as Anselm says in the book on the virginal conception, and so original sin is a sort of privation. But privation is opposed to habitude. Therefore original sin is not habitude.
Further, actual sin has more of the notion of fault than original, inasmuch as it has more of the notion of the voluntary. But actual habitude of sin does not have the notion of fault; otherwise it would follow that a man sleeping, culpably sins. Thefore no original habitude has the notion of fault.
Further, in badness, act always precedes habitude, for bad habitude is never infused, but acquired. But original sin is not preceded by any act. Therefore original sin is not habitude.
But contrariwise is what Augustine says in the book on the baptism of infants, that due to original sin little children are capable of craving [concupiscibili], though they are not actually craving [concupiscentes]. But capability [habilitas] is called according to some habitude. Therefore original sin is habitude.
I reply that it must be said that, as was said above, habitude is twofold. One is that by which power is inclined to act, as kinds of knowledge [scientiae] and virtues are called habitudes. And in this way, original sin is not habitude. In another way, habitude is said to be a disposition of a nature composed of many things, according to which it has itself [se habet] either well or badly toward something, and especially according as the disposition has been turned as it were into nature, as is obvious from illness and health. And in this way original sin is habitude. For it is a sort of disordered [inordinata] disposition coming from the dissolution of that harmony in which the notion of original justice consisted, just as bodily illness is a sort of disordered [inordinata] disposition of body, according to which the equality in which the notion of health consists is dissolved. Whence original sin is called languor of nature.
To the first it must be said that, as illness of body has something of privation, inasmuch as equality of health is removed, and has something positive, to wit the humors themselves being disposed disorderedly [inordinata dispositos], so too original sin has privation of original justice, and with it disordered disposition [inordinatam dispositionem] of the parts of the soul. Whence it is not pure privation but a sort of corrupt habitude.
To the second it must be said that actual sin is a sort of disordering [inordinatio] of act, whereas original sin, since it is sin of nature, is a sort of disordered disposition [inordinata dispositio] of nature itself, which has the notion of fault inasmuch as it is derived from the first parent, as was said. Now this kind of disordered disposition of nature [inordinata dispositio naturae] has the notion of habitude, but disordered disposition of act [inordinata dipositio actus] does not have the notion of habitude. And because of this original sin is able to be habitude, but not actual sin.
To the third it must be said that that objection proceeds from the habitude by which power is inclined to act, but original sin is not such a habitude. Although even from original sin some inclination to disordered act [actum inordinatum] follows, not directly, but indirectly, to wit, through removal of the impediment [remotionem prohibentis], that is, original justice, which impeded disordered change [inordinatos motus], just as also from bodily illness there follows inclination to disordered bodily changes [motus corporales inordinatos]. Nor ought it to be said that original sin is infused habitude, nor acquired, save by the act of the first parent rather than the act of this person, but it is innate through defective origin.
[St. Thomas Aquinas, ST 2-1.82.1, my translation. The Latin is here, the Dominican Fathers translation is here.]
It's important to grasp that original sin is not sin in a strictly proper sense, i.e., in the sense that the person who has it has thereby sinned; rather, it is the sin of the whole human race, in the first parent as the head of the human race. In that parental sin, original justice, which protected from actual disorder, was lost, with the result that all descended human beings are disordered in their desires.
Besides the importance of original sin itself, one of the important points emphasized here is that the infused vs. acquired distinction is not exhaustive for habitudes; original sin is neither infused nor acquired by the person who has it, but is a result of a defective origination of the person, namely, being generated when original justice has been lost. This makes original sin a natural habitude like congenital illness rather than a rational habitude like science or virtue, although since it is a disordered state of our rational ability to organize our inclinations, disordered acts follow from it, which makes vice possible and, indeed, in the long run inevitable where there is nothing to counteract it.