Leprosy makes one unclean, not because it was contagious (see Leviticus 13:1–14:57)—for then people whose whole bodies were affected would not have been pronounced clean (Leviticus 3:12–13)—but because its disfiguring effect (ulcers on the face and other body parts, even causing parts of the body to fall off) was an image of sin’s spiritual impurity and uncleanness (compare Leviticus 13:12 with Isaiah 1:6). The cleansing of the leper, shown in the Greek verb katharizein—through sacrifice and ritual washing, and by Jesus’ own healing power—points forward to the cleansing power of Christ’s blood. Jesus has the power to give the leper ritual purity (Matthew 8:2) if he desires to do so (if you will
—see John 5:21; Matthew 17:24).
It is not in the first place the nature of the illness itself, but much rather the fact that it makes one unclean in terms of the Levitical law that is the core issue when it comes to leprosy. It is the kind of illness that, because of its symptoms, was particularly suited to personify the spiritual impurity of sin, for the leprosy most common in Palestine manifested in ulcers on the face and other body parts, which devours flesh and even causes parts of the body to fall off. It is not clear, however, whether this particular kind of leprosy was even contagious. After all, the divine prescriptions in the Law regarding leprosy (see Leviticus 13:1–59 and Leviticus 14:1–57) were never intended to stop the spread of disease. If this had been the case, people whose whole bodies had been affected thereby would not have been pronounced clean (see Leviticus 13:12). The purpose rather was to highlight the sinfulness and uncleanness of mankind before God through the manifestation of leprosy (compare Leviticus 13:12 with Isaiah 1:6). By witnessing the manifestation of leprosy, Israel had to be reminded of man’s fallen state before his Creator. By pronouncing a healed leper clean through sacrifice and ritual washing, the priest pointed to the blood of Christ.1
Being made clean from leprosy is in the Greek always indicated by a form of the word katharizein, which means being purified. This indicates that it is a ritual purity that is at issue here in Matthew 8:2. That Jesus has the power and authority to purify is certain, but the only question here is whether he desires to do so: (if you will
—see also in this regard John 5:21 and John 17:24).2
2 And behold, a leper came to him and knelt before him, saying, “Lord, if you will, you can make me clean.”