Oktzim, Chapter Three, Mishnah Three
1) The carrion of an unclean beast at all places, and of a clean bird in villages, need intention but do not need to be rendered susceptible.
2) The carrion of a clean beast in all places, and that of a clean bird and also fat in the market places, require neither intention nor to be rendered susceptible.
3) Rabbi Shimon says: also [the carrion of] the camel, rabbit, hare or pig.
Section one: The foods listed in this section are not generally eaten, at least according the first opinion. Therefore, to defile one would need to think about them as food. However, since they are already impure they don’t need to be rendered susceptible to impurity.
Carrion of an unclean beast, for instance horse carrion, defiles. This mishnah really didn’t need to teach this, because it is stated clearly in the Torah. In order to find an innovation in the mishnah the talmud notes that generally less than an olive’s worth does not defile because it is too small of an amount. The mishnah is understood as innovating that if 1/2 of an olive’s worth of carrion from an unclean beast joins together with a 1/2 of an olive’s worth of other unclean food, the two halves join together to defile in the same was as would impure food.
If one doesn’t think about eating this carrion, it’s not considered food because most people don’t consider unclean beasts to be food.
The carrion of a clean bird is forbidden to eat but it doesn’t defile except when in the gullet (see Toharot 1:1). In the villages where there aren’t a lot of people evidently most people wouldn’t bother eating bird carrion. Therefore, one would specifically have to think about eating it for it to defile.
Section two: Carrion of a clean beast is forbidden but since most people do eat such animals, it causes food impurity even if one doesn’t specifically think about using it for food. It also doesn’t need to be made susceptible because carrion is already impure.
In market places people will be found to buy carrion of clean birds and forbidden fat. Therefore, they don’t need to be thought of as food to defile. And since they are already impure, they don’t need contact with liquid to become susceptible.
Section three: Rabbi Shiomon says that people eat camel, hare, rabbit and pig. Therefore, the same rules that apply to carrion of clean meat (cow, sheep, goats) apply to them.