Leviticus 21:11 "Neither shall he come close to any dead body"

A High Priest must always keep himself in a state of purity.

Unlike a regular priest who may become impure from contact with his closest relatives who have died (see Negative Mitzvah 166), a High Priest may not become impure from contact with any dead body.

This Negative Mitzvah cautions a High Priest to be careful not to be under the same roof as a dead body, even if it is one of his seven closest relatives.