Makhshirin, Chapter Six, Mishnah Six
1) The following cause uncleanness and also susceptibility [to uncleanness];
2) The flow of a zav, his spittle, his semen and his urine;
3) A quarter-log of blood from a corpse, and the blood of a menstruant.
4) Rabbi Eliezer says: semen does not cause susceptibility.
5) R. Elazar ben Azariah says: the blood of a menstruant does not cause susceptibility.
6) Rabbi Shimon says: the blood of a corpse does not cause susceptibility, and if it fell on a gourd, he can scrape it off, and it remains clean.
Section one: The fluids in this mishnah are both impure and they make food susceptible to impurity. Two for the price of one!
Section two: A zav is a person with some sort of unnatural genital flow (i.e. not semen and not menstrual blood). If a zav emits the fluids listed here they cause food to become impure immediately, even if it hadn’t previously come into contact with water.
Section three: Both of these types of blood cause impurity and susceptibility.
Section four: Rabbi Eliezer holds that semen does not cause susceptibility. Therefore food will have to first be made susceptible for contact with semen to cause it to be impure.
Section five: Rabbi Elazar ben Azariah hold that menstrual blood does not cause susceptibility.
Section six: Rabbi Shimon is even more lenient and he holds that even the blood of a corpse doesn’t cause susceptibility.
The Talmud explains that in all of these cases since the substance is referred to by two words (shikhvat zera is the Hebrew for semen, dam niddah for menstrual blood and dam hamet for blood of a corpse) the fluid doesn’t cause susceptibility. Only fluids referred to by one wordblood, milk, etc. cause susceptibility.