Commentary for Tahorot 5:1
הַשֶּׁרֶץ וְהַצְּפַרְדֵּעַ בִּרְשׁוּת הָרַבִּים, וְכֵן כַּזַּיִת מִן הַמֵּת וְכַזַּיִת מִן הַנְּבֵלָה, וְעֶצֶם מִן הַמֵּת וְעֶצֶם מִן הַנְּבֵלָה, וְגוּשׁ מֵאֶרֶץ טְהוֹרָה וְגוּשׁ מִבֵּית הַפְּרָס, גּוּשׁ מֵאֶרֶץ טְהוֹרָה וְגוּשׁ מֵאֶרֶץ הָעַמִּים, שְׁנֵי שְׁבִילִין, אֶחָד טָמֵא וְאֶחָד טָהוֹר, הָלַךְ בְּאַחַד מֵהֶם וְאֵין יָדוּעַ בְּאֵיזֶה מֵהֶן הָלַךְ, הֶאֱהִיל עַל אַחַד מֵהֶם וְאֵין יָדוּעַ עַל אֵיזֶה מֵהֶן הֶאֱהִיל, הִסִּיט אֶת אַחַד מֵהֶם וְאֵין יָדוּעַ אֵיזֶה מֵהֶם הִסִּיט, רַבִּי עֲקִיבָא מְטַמֵּא, וַחֲכָמִים מְטַהֲרִים:
A vermin and a frog in the public domain, and similarly an olive's volume of a dead body and an olive's volume of an animal carcass, or a bone from a dead body and a bone from an animal carcass, or a clod [of dirt] from a pure land and a clod from a plowed cemetery, a clod from a pure land and a clod from the lands of the nations [i.e. outside Israel], or two paths one impure and one pure, if someone walked down one of them and he does not know which one he walked down, or [in passing by] he covered over one of them [i.e. over the olive's volume of the dead body or of the animal carcass] and he does not know which one he covered over, or he moved one of them [i.e. one of the two bones, or one of the two clods of dirt] and he does not know which one he moved, Rabbi Akiva declares him impure, but the Sages declare him pure.
Bartenura on Mishnah Tahorot
English Explanation of Mishnah Tahorot
And so also [if there was there] an olive's bulk of a corpse and an olive's bulk of carrion,
A bone of a corpse and a bone of carrion;
A clod of clean earth and a clod from a doubtful grave area
A clod of clean earth and a clod from the land of the Gentiles,
Or if there were two paths, the one unclean and the other clean, and a man walked through one of them but it is not known which,
Or if overshadowed one of them but it is not known which, or he shifted one of them but it is not known which:
Rabbi Akiva rules that he is unclean, But the sages rule that he is clean.
Today's mishnah brings up a series of cases in which one of two possible things occurred one which would have caused him to be impure and one in which he was not impure. In the end of the mishnah we can see that the rabbis and Rabbi Akiva disagree as to whether or not the person is pure or impure.
Section one: A dead sheretz causes impurity but a dead frog does not because it is not one of the defiling creepy crawly things listed in Leviticus 11:29.
Section two: Flesh from a corpse defiles in all ways, but flesh from carrion of an animal defiles only by touch and by carrying, not by overshadowing.
Section three: Bone of carrion is pure. Bone from a corpse defiles through contact and carrying.
Sections four and five: Earth from an area that might have been a graveyard defiles through contact and carrying, as does dirt from outside of the land of Israel.
Section seven: This section refers to all of the sections above.
Section eight: Rabbi Akiva says that doubtful cases of impurity in the public domain are pure only if the person didn't perform any action. But in the cases listed here the person moved, carried or overshadowed one of the objects that might have been impure, and therefore he is impure.
The other sages disagree doubtful cases of impurity are always pure in the public domain.