When a person became unclean, someone who was clean was to dip hyssop into the water and sprinkle him (verses 18 and 19, "thrown" in verse 13). To fail to be cleansed in this way is a serious matter, because twice the person who fails to be purified is described as being excluded from the covenant people (verses 13 and 20).
This is the background of baptism in the New Testament, and suggests that the proper mode is by sprinkling, not by pouring or immersion. This is not to suggest that mode of baptism is a salvific issue. Rather, it is merely to point out that Baptists are wrong when they make immersion a salvific issue. Since I myself was immersed, I would never say that a person who hasn't been sprinkled is not a legitimate Christian.
No doubt someone is saying that this doesn't settle the issue. And I would be quick to agree. By itself, this argument is certainly not conclusive. However, added to what I have said before (here and here), I think it contributes to a solid case.