Leviticus 14:5

5-9. the priest shall command that one of the birds be killed ... over running water--As the blood of a single bird would not have been sufficient to immerse the body of another bird, it was mingled with spring water to increase the quantity necessary for the appointed sprinklings, which were to be repeated seven times, denoting a complete purification. (See 2Ki 5:10; Psa 51:2; Mt 8:4; Lu 5:14). The living bird being then set free, in token of the leper's release from quarantine, the priest pronounced him clean; and this official declaration was made with all solemnity, in order that the mind of the leper might be duly impressed with a sense of the divine goodness, and that others might be satisfied they might safely hold intercourse with him. Several other purifications had to be gone through during a series of seven days, and the whole process had to be repeated on the seventh, ere he was allowed to re-enter the camp. The circumstance of a priest being employed seems to imply that instruction suitable to the newly recovered leper would be given, and that the symbolical ceremonies used in the process of cleansing leprosy would be explained. How far they were then understood we cannot tell. But we can trace some instructive analogies between the leprosy and the disease of sin, and between the rites observed in the process of cleansing leprosy and the provisions of the Gospel. The chief of these analogies is that as it was only when a leper exhibited a certain change of state that orders were given by the priest for a sacrifice, so a sinner must be in the exercise of faith and penitence ere the benefits of the gospel remedy can be enjoyed by him. The slain bird and the bird let loose are supposed to typify, the one the death, and the other the resurrection of Christ; while the sprinklings on him that had been leprous typified the requirements which led a believer to cleanse himself from all filthiness of the flesh and spirit, and to perfect his holiness in the fear of the Lord.

Robert Jamieson, A. R. Fausset & David Brown, 1871 · public domain

Verse 5 Over running water - Literally, living, that is, spring water. The meaning appears to be this: Some water (about a quarter of a log, an eggshell and a half full, according to the rabbins) was taken from a spring, and put into a clean earthen vessel, and they killed the bird over this water, that the blood might drop into it; and in this blood and water mixed, they dipped the instrument before described, and sprinkled it seven times upon the person who was to be cleansed. The living or spring water was chosen because it was purer than what was taken from pits or wells, the latter being often in a putrid or corrupt state; for in a ceremony of purifying or cleansing, every thing must be as pure and perfect as possible.

Adam Clarke, 1831 · public domain