Niddah 2 - Presumption of Ritual Purity
A woman who discharges uterine blood becomes ritually impure, and transmits the impurity to the foods she is working with. That is true even if the blood has not yet descended through the vaginal canal to the outside of her body. Thus, if a woman performs an internal examination by wrapping a cloth around her finger and by carefully wiping it against the sides of the vaginal canal, and finds blood on the cloth, it is possible that she has been impure for some time before that.
Shammai says that nevertheless only those foods that she touches from now on become impure. That is because there is a rule of preserving the status quo: the woman was pure before, so we can assume that she only becomes impure from the moment when she actually sees the blood. Hillel, however, says that we go all the way to the previous examination, even days back, and declare all foods that she touched potentially impure - since the blood may have appeared right after her previous examination. Hillel agrees to the principle of status quo, but in this case there is something that weakens the presumption - she sees actual blood.
The Sages take an intermediate position, namely, that the food she touches are declared impure retroactively - either to the last examination, or twenty four hours back, whichever is shorter. The Sages agree in principle with Hillel, but they say that the blood cannot remain in the vaginal canal for longer than twenty four hours.
Art: Esaias Boursse - A Woman Seated at a Table Cutting a Slice of Cheese