Taken literally, I can't see any explanation except that it is some shamanic recipe dating from the bronze age, and medically completely bogus if not downright dangerous.
Does it have an interpreted meaning? How do Bible literalists justify this?
Small excerpt, there are pages and pages of ritual...
4 the priest shall order that two live clean birds and some cedar wood, scarlet yarn and hyssop be brought for the person to be cleansed. 5 Then the priest shall order that one of the birds be killed over fresh water in a clay pot. 6 He is then to take the live bird and dip it, together with the cedar wood, the scarlet yarn and the hyssop, into the blood of the bird that was killed over the fresh water. 7 Seven times he shall sprinkle the one to be cleansed of the defiling disease, and then pronounce them clean. After that, he is to release the live bird in the open fields.