Parah, Chapter 12, Mishnah 7. This chapter deals with the sprinkling of the mei chatas, the waters of the parah adumah, and with the unusual stringencies that apply to everything connected with the chatas. Our mishnah turns from the ezov and the water to the person: what becomes of a man who had prepared himself in taharah for the chatas, and then his hands became tamei?
The case
The mishnah states: "Hatahor lechatas shenitme'u yadav", one who is tahor for the chatas whose hands became tamei. Hands can contract tumah on their own in several ways. He may have touched something that is a rishon letumah, a first degree of tumah, which renders the hands that touch it tamei. Or he may have handled kisvei hakodesh, holy writings, which the Chachamim decreed render the hands tamei, a tumas yadayim that is only miderabbanan.
The ruling
In the ordinary laws of tumah, such a case leaves the hands tamei while the rest of the body remains tahor. Not so with the chatas. The mishnah rules "nitma gufo": his entire body has become tamei. The tumah that settled on his hands alone is treated, for the chatas, as though it had taken hold of the whole man, and he is no longer fit to occupy himself with the mei chatas.
And on to a hundred
The mishnah continues: "umetamei es chaveiro, vechaveiro es chaveiro, afilu hein meiah". Having become tamei in his whole body, he now conveys that tumah to his fellow who was tahor for the chatas, rendering that man's entire body tamei as well; that second man conveys it to a third, and so onward, even through a hundred men. Here again we meet the great chumra of the chatas: the tumah is handed along from one to the next without ever losing its strength, no matter how long the chain becomes.
The lesson of the mishnah is how far the Torah's stringency reaches in the service of the parah adumah. A tumah that would elsewhere be confined to a man's hands, and in one of the cases is only a rabbinic tumah to begin with, is enough here to disqualify him entirely and to disqualify everyone who touches him after him.