Commentaire sur Parah 12:7

If the hands of a man who was clean for the hatat waters became unclean, his body also becomes unclean, and he conveys uncleanness to his fellow, and his fellow to his fellow, even if they be a hundred. If the hands of a person who is pure enough to perform the hatat ritual become unclean, the uncleanness spreads to his whole body. This is not true with other levels of purity restrictions. In other cases, a person's hands can become impure without the rest of his body also becoming impure. Furthermore, as we saw in yesterday's mishnah, the chain of impurity vis a vis the hatat continues down the line. So if he touches a person and that person touches another person, etc., the impurity is continually transmitted and anyone who is in the chain will not be allowed to perform the hatat ritual. This too is not true in other cases, where eventually the impurity is lessened sufficiently such that it no longer defiles at all. Again, we can see that the rules regarding the hatat are stricter than the normal rules with regard to impurity.