Desecration of the Host

687. – After showing the dignity of this sacrament, the Apostle now rouses the faithful to receive it reverently. First, he outlines the peril threatening those who receive unworthily; secondly, he applies a saving remedy (v. 28).

688. – First, therefore, he says, Therefore, from the fact that this which is received sacramentally is the body of Christ and what is drunk is the blood of Christ, whoever eats this bread or drinks the cup in an unworthy manner will by guilty of profaning the body and blood of the Lord. In these words must be considered, first, how someone eats or drinks unworthily. According to a Gloss this happens in three ways: first, as to the celebration of this sacrament, namely, because someone celebrates the sacrament in a manner different from that handed down by Christ; for example, if he offers in this sacrament a bread other than wheaten or some liquid other than wine from the grape of the vine. Hence it says in Lev (10:1) that Nadab and Abihu, sons of Aaron, offered before the Lord “unholy fire, such as he had not commanded them. And fire came forth from the presence of the Lord and devoured them.”

689. – Secondly, from the fact that someone approached the Eucharist with a mind not devout. This lack of devotion is sometimes venial, as when someone with his mind distracted by worldly affairs approaches this sacrament habitually retaining due reverence toward it; and such lack of devotion, although it impedes the fruit of this sacrament, which is spiritual refreshment, does not make one guilty of the body and blood of the Lord, as the Apostle says here. But a certain lack of devotion is a mortal sin, i.e., when it involves contempt of this sacrament, as it says in Mal (1:12): “But you profane it when you say that the Lord’s table is polluted and its food may be despised.” It is of such lack of devotion that the Gloss speaks.

690. – In a third way someone is said to be unworthy, because he approaches the Eucharist with the intention of sinning mortally. For it says in Lev (21:23): “He shall not approach the altar, because he has a blemish.” Someone is understood to have a blemish as long as he persists in the intention of sinning, which, however, is taken away through penitence. By contrition, indeed, which takes away the will to sin with the intention of confession and making satisfaction, as to the remission of guilt and eternal punishment; by confession and satisfaction as to the total remission of punishment and reconciliation with the members of the Church. Therefore, in cases of necessity, as when someone does not have an abundance of confessors, contrition is enough for receiving this sacrament. But as a general rule, confession with some satisfaction should precede. Hence in the book on Church Dogmas it says: “One who desires to go to communion should make satisfaction with tears and prayers, and trusting in the Lord approach the Eucharist clean, free from care, and secure. But I say this of the person not burdened with capital and mortal sins. For the one whom mortal sins committed after baptism press down, I advise to make satisfaction with public penance, and so be joined to communion by the judgment of the priest, if he does not wish to receive the condemnation of the Church.”

https://dhspriory.org/thomas/english/SS1Cor.htm#117

i) A basic problem with his interpretation is that, in context, the sin resulting in mortal illness isn't desecration of the Host but dishonoring fellow Christians at the agape feast. 

ii) That doesn't mean it's impossible to commit sacrilege. If you immerse a crucifix in urine, that's sacrilegious, not because a crucifix is intrinsically holy, but because there's the intent to express contempt for Christianity by using a symbol that represents the Christian faith. 

Conversely, when priests trample on the crucifix (or icon of Christ) in Silence (2016) to spare Japanese Christians from torture, that's not sacrilegious because it's just a symbol, and their intention is not to profane the faith, but to save the innocent from brutalization. 

iii) We might also draw a distinction between sacrilege and desecration. If Crusaders use a mosque as a latrine, that's a deliberate act of desecration, but unless Islam is true, it's not sacrilegious–for it doesn't profane the true God.