Hyssop is a small bushy plant used for the sprinkling of blood on the altar or objects to be sanctified (see Psalm 51:7). Different kinds of plants seem to be given this name. In the Gospel of John (John 19:29) a hyssop branch with a sponge was used to give vinegar to Christ on the cross. Here, hyssop was used to sprinkle the person seven times before he was pronounced clean. This parallels the Day of Atonement, where the blood was sprinkled seven times.
6 He shall take the live bird with the cedarwood and the scarlet yarn and the hyssop, and dip them and the live bird in the blood of the bird that was killed over the fresh water.