Parah, Chapter 12, Mishnah 6. This chapter is devoted to the ezov, the hyssop with which the mei chatas (the waters of the parah adumah) are sprinkled, and to the many stringencies that the Torah and the Chachamim attached to everything connected with the chatas. Our mishnah asks what happens when the ezov itself is tamei.
The mishnah opens: "Hamazeh be'ezov tamei", one who sprinkles with an ezov that has become tamei. The ruling then divides according to the size of that ezov.
An ezov of a k'beitzah
"Im yesh bo k'beitzah, hamayim pesulim vehaza'aso pesulah": if the tamei ezov has the volume of an egg, the water is unfit and the sprinkling is unfit. The reason lies in the shiur that governs tumas ochlin: a food that is tamei conveys its tumah further only when it has the measure of a k'beitzah. Here the ezov had been picked as food, so it is susceptible to tumah as a food, and since it holds a k'beitzah it passes its tumah along to whatever it touches. Dipped into the mei chatas, it renders the water tamei, and once the water is pasul, the sprinkling done with it accomplishes nothing.
An ezov smaller than a k'beitzah
"Ein bo k'beitzah, hamayim kesherim vehaza'aso pesulah": if the ezov does not hold a k'beitzah, the water remains fit, but the sprinkling is still unfit. Two separate points are at work. Because the ezov lacks the measure of a k'beitzah, it has no power to transmit its tumah, so the water in which it was dipped stays kosher and may be used for another sprinkling. Yet the sprinkling that was performed with this ezov is invalid, because the ezov used for the mei chatas must itself be tahor, and it must have been picked for the purpose of sprinkling. This ezov was picked as food, not for the chatas, and it is tamei besides. On both counts it is disqualified for the avodah of sprinkling.
A chain of tumah without end
The mishnah closes: "umetamei es chaveiro, vechaveiro es chaveiro, afilu hein meiah". The tamei ezov renders another ezov tamei, and that second one renders a third tamei, and so on, even through a hundred. This is one of the well known chumros of the chatas: in the ordinary laws of tumah each remove is weaker than the one before it until the chain gives out, but where the chatas is concerned the tumah is passed along from one to the next without weakening, however long the line grows.
So our mishnah teaches us both the practical difference that the shiur of a k'beitzah makes for the water, and the independent requirement that the ezov be tahor and designated for sprinkling if the haza'ah is to be valid at all.