Or HaChaim on Leviticus 11:40/אור החיים על ויקרא י״א:מ׳

1 והאכל מנבלתה, anyone eating of its carcass, etc. According to Torat Kohanim the only reason this verse was written is to inform us that the size of an animal which confers ritual impurity on someone carrying or touching its carcass is the same as that of someone eating of it, i.e. the size of an olive. The verse does not intend to convey the meaning that a reader who did not read any of the aforegoing would have derived from it, namely the prohibition of eating from such a carcass and that swallowing it confers impurity. The reason that we cannot understand the verse in that way is that we already have a clear verse in Leviticus 22,8: "he must not eat it on pain of becoming impure through it." The word בה in that verse is an exclusion and means that only the carcass of a pure bird which died of natural causes confers such impurity as soon as one swallows it but not the swallowing of parts of a mammal which dies of natural causes rather than ritual slaughter. Dead birds do not confer impurity through contact; therefore the Torah had to tell us that if the bird was a pure bird it does confer impurity as soon as it reaches one's esophagus and if a priest ate from that bird he is disqualified from performing his duties unless he purifies himself. Why did the Torah not spell out such a regulation but depended on our deriving it through exegesis? Perhaps the Torah intended to teach us numerous halachot from a single verse, something that would have been difficult if the verse had only stated that the carrion of a clean bird is capable of conferring impurity by one's swallowing it. We are taught in Chulin 71, for instance, that if the meat of a dead swarming thing has deteriorated to the extent that a dog would reject it as unfit to eat, it no longer confers impurity. This halachah is derived by Tossaphot in Bechorot 23. We have a verse in Kings II 9,10 where Izzevel, wife of king Achav, is described as being eaten by the dogs, not being buried. We note that consumption of meat by dogs is described by the term אוכל the same term used for humans consuming food. Seeing that it is the intention of our verse to establish a linkage between eating and impurity, it is only logical that other halachot of that nature may be derived from this verse.