We have learned in the Mishna (79a) that when the majority of the Jewish people are tameh (ritually defiled by contact with a dead body) then the korban Pesah will be brought and eaten anyway. In such a case there is no need to bring a Pesah sheni (see 9:6-14), which is reserved for an individual who cannot bring the Passover sacrifice in the proper time.
We usually think of the Jewish people as a single unit, but according to some we should look at it as divided up into the twelve tribes. Our Gemara quotes a baraita that brings the opinions of Rabbi Shimon and Rabbi Yehuda on this matter.
Rabbi Shimon says that if the majority of a single shevet – just one of the tribes – is tameh, they will be able to bring the korban Pesah at its proper time even though they are tameh, while the rest of the shevatim will bring it at the same time, but separately, keeping the normal rules of tuma and tahara (ritual impurity and purity). The Gemara explains that Rabbi Shimon understands that each one of the shevatim is considered a kahal – a community – unto itself.
Rabbi Yehuda says: Even if one tribe is impure and all the rest of the tribes are pure, all the tribes may perform the ritual of the Paschal lamb in a state of ritual impurity, as a communal offering is not divided. The Gemara explains that Rabbi Yehuda holds that one tribe is called a community, and since an entire community is impure, it is considered as though half the Jewish people were pure and half were impure. And a communal offering is not divided. Therefore, all of them may perform the ritual of the Paschal lamb in a state of ritual impurity.
The Gemara explains Rabbi Yehuda’s position as agreeing with Rabbi Shimon that a single shevet is considered a kahal. He believes, however, that such a community is so important that it balances the entire rest of the Jewish people. Since we perceive the two as being equal in weight we do not split the communal sacrifice, rather we bring it be-tuma.
The discussion about the status of a single shevet has its basis in Massekhet Horayot and the question of how to deal with a case where “the entire Jewish people” commit a sin (see 4:13-21). Should we view a single shevet as a distinct community and that the rules that apply to the community apply to them, or do we perceive the shevet simply as part of the larger community – merely as a large number of individuals, but not a community unto themselves.