Niddah, Chapter 10, Mishnah 7. This Mishnah deals with a woman who has given birth and is now in her days of dam tohar, the stretch of time following childbirth during which blood that leaves her body does not render her a niddah but is halachically pure blood. The question is how far that purity extends when kodshim, consecrated food and offerings, are involved.
The original ruling
"Barishona hayu omrim": the earlier ruling handed down by the Sages imposed no limitation on such a woman whatsoever. The Mishnah's term for her is "hayosheves al dam tohar", one who is sitting in her days of pure blood. In that condition she was permitted "haysa me'arah mayim lepesach", to pour out the water used for the rinsing and preparation of the korban Pesach. And why not? The blood she sees during this stretch of time carries no tumah at all, her hands never come into contact with the water she is pouring, and the kli from which the water flows is not rendered tamei by her. With nothing tamei about her and nothing of hers touching the water, there was no ground for excluding her from this task.
They retracted and were stringent
"Chazru lomar": the Sages afterward reversed themselves and adopted a stringency "lekadshim", in matters involving consecrated food. Her standing, they now taught, is "kemaga tmei meis", like that of a person who touched someone who had already become tamei from a corpse; and this ruling is "kedivrei Beis Hillel", the position of Beis Hillel. Such contact generates only a derived, second-level tumah, but even a tumah of that modest rank suffices to curtail her part in handling the water needed for the Pesach offering.
Beis Shamai
Beis Shamai add: "af kitmei meis", even like a tmei meis himself. In their opinion her status is not the derived one but the primary one: she is reckoned as someone who absorbed tumah straight from a corpse. The practical outcome is that in anything pertaining to kadshim she is excluded altogether, having no dealings whatsoever with the water, and none with the vessel either.