Aside from “pushing aside” Shabbat, the korban Pesah also “pushes aside” the rules of ritual defilement in a case where the majority of the Jewish people find themselves to be in that situation, a rule know as tuma hutra be-tzibur. The Gemara discusses three level of tuma– of ritual defilement:
Tumat met – someone whose ritual defilement stems from contact with a dead body
Tumat zav – someone suffering from a venereal disease.
Tumat tzora’at – someone who is a metzora (commonly translated as leprosy)
It is difficult to clearly categorize these different tumot as far as their respective levels are concerned. Although a detailed list appears in the first perek of Massekhet Kelim, nevertheless there are many issues involved – which have the greatest ability to spread tuma, how each of them spreads tuma (e.g. physical contact, lifting, entering a house, etc.) and even the size of the thing that can become tameh. Our Gemara teaches that tumat met is the least severe of the three, as he would only be restricted from the mahane shehina that surrounds the Mishkan. The zav cannot enter the next section, the mahane levi’a, where the Tribe of Levi lived. Tumat tzora’at is the most severe with regard to the encampment of the Jewish people in the desert, as the metzora is banned even from mahane yisrael, which is where the general populace lived.
This is an example of how the Sages make use of the encampment of the Jews in the desert as a model for contemporary halakhot. Certain laws that were taught in the context of the desert retain that same language in the discussions of the Talmud, even as the rules are made to apply to more modern times. In up-to-date terms, mahane shehina represents the Temple itself, mahane levi’a represents the Temple mount and mahane yisrael is the city of Jerusalem – and, according to many rishonim it includes all walled cities in Israel.