It is the highest grade of uncleanness, or defilement, known to man and is contracted by having either directly or indirectly touched, carried or shifted a dead human body,[1] or after having entered a roofed house or chamber where the corpse of a Jew is lying (conveyed by overshadowing).
[15] A Jew who is descended from a line of the priestly stock known as Kohen is not allowed to intentionally come into contact with a dead body, nor approach too closely to graves within a Jewish cemetery.
[18] An ordinary priest of Aaron's lineage is, however, permitted to contract corpse uncleanness for any of his seven closest relatives that have died (father, mother, brother, unwedded sister, son, daughter, or wife),[19] including a married sister by a rabbinic injunction.
[20] Jewish priests were especially susceptible to contracting corpse uncleanness, due to the unmarked graves in foreign lands.
Since they were required by a biblical injunction to eat their bread-offering (Terumah) in a state of ritual purity, and they could hardly know if they had trampled upon an unmarked grave, this prompted the early rabbis to decree a general-state of defilement upon all foreign lands.
[21] Public roads in the land of Israel, however, were assumed to be clean from corpse defilement, unless one knew for certain that he had touched human remains.
Human corpse uncleanness requires an interlude of seven days, accompanied by purification through sprinkling of the ashes of the Parah Adumah, the red heifer.
[23] However, the law is inactive, since neither the Temple in Jerusalem nor the red heifer are currently in existence, though without the latter, a Jew is forbidden to ascend to the site of the former.