Toharot, Chapter Two, Mishnah Eight
1) If one eats food with second [degree uncleanness he must not work in an olive-press.
2) Common food that was prepared under conditions proper to the cleanness of consecrated food is still regarded as common food.
a) Rabbi Elazar bar Zadok says: it is regarded as terumah to convey uncleanness at two removes and to render terumah invalid at one additional remove.
Section one: A person who eats food with second degree uncleanness gets second degree uncleanness, as we learned in mishnah two. Therefore, he shouldn’t work in an olive press because when he touches the olive oil, he will convey to it first degree uncleanness, because it is a liquid.
Section two: The mishnah again makes reference to common food (hullin) that was prepared under the conditions of purity that are normally used for holy food. Such food is still considered to be hullin, and therefore there is no such thing as third degree impurity. Food that had contact with such food that had second degree impurity can still be eaten.
Rabbi Elazar bar Zadok says that it is like terumah. First and second degree are impure, third degree is invalid but doesn’t convey impurity onward. In other words, by treating this common food as if it was holy, it does attain added holiness.