Leviticus 11:24

and by them you would be defiled: whosoever toucheth their carcase shall be unclean until evening.

Through these you will become ritually impure Some people say that the winged creeping beings [:32] and the birds that are to be detested [:13] are meant here. Others are of the opinion that these has the force of “the following”, and that all four-legged animals that move on their paws [:27] are meant. I believe that these includes all the animals that have been mentioned above.

you will become ritually impure The verb is in the reflexive [hitpa‘el] form. Its ṭet is doubled to include the missing tav , like the daled in “he heard the Voice communing [Hebrew: middabber] with him” [Numbers 7:89] and “the one purifying himself” [14:8].

will be ritually impure until the evening Scripture employs brevity, not specifying that he first must immerse himself in water.

AND BY THESE YE SHALL BECOME UNCLEAN. “By these which are to be mentioned further on in the section, ye shall become unclean. Here Scripture teaches you that the carcass of an animal unfit for food conveys impurity, and in the subject at the end of the section it explains [the law] concerning the carcass of an animal fit for food.” This is Rashi’s language. But if so, [the question arises], why did Scripture divide them and why did it treat them singly, animals, were discussed in Verses 24-28 in detail, with separate verses for each category of these unclean animals, when one general statement could have covered all subjects? when it should rather have said [in one general statement], “whosoever touches the carcass of any animal shall be unclean until the evening, and whosoever carries any of its carcass shall wash his clothes and be unclean until the evening?”
The correct interpretation is that here Scripture mentioned the carcass of them without qualification and did not mention “death” in connection with them, thus teaching that [the law of] ritual slaughtering [required to render animals fit for food] does not apply to them [the unclean animals mentioned here], and whoever touches them when they are no longer alive [whether they died by themselves or even if they were ritually slaughtered], becomes impure. But further on it states, And if any beast, of which ye may eat, die, thus teaching that if it [an animal fit for food] was ritually slaughtered, it does not convey impurity [to a person who touches it]. Now the meaning of the section in mentioning [here]: And by these ye shall become unclean, is to state that all things referred to above — namely, fishes, fowls, and grasshoppers — do not have this [law of] impurity, but only those which He is about to mention further on, these being Every beast … which is not cloven-footed, nor cheweth the cud. It is with reference to these that He states, they are unclean to you; every one that toucheth them shall be unclean with the uncleanness mentioned [in the preceding Verses 24-25: whosoever toucheth the carcass of them shall be unclean until the even. And whosoever beareth aught of the carcass of them shall wash his clothes, and be unclean until the even]. Then He continues by stating that any beast which walks upon its paws shall also convey impurity until the evening to a person who touches its carcass, and that a person who …

ולאלה תטמאו, “if you touch any of the following (dead carcasses) you will become ritually impure.” The list of such animals follows.