The Torah in Parashat Naso (Bamidbar 5:2) commands that all those who have a status of Tum’a (impurity) must be sent away from the area of the Mishkan. This verse establishes a command requiring one to distance himself from the area of the Bet Ha’mikdash when in a state of Tum’a.
In explaining the reason behind this Misva, the Sefer Ha’hinuch writes that the Sages understood that Tum’a adversely affects the “Ko’ah Ha’nefesh Ha’sichlit” – the power of one’s mind, his intellectual capacity. It causes a degree of confusion, as though creating a “blockage” between him and what the Sefer Ha’hinuch calls “the fountains of the intellect.” The basis for this notion is the verse in the Book of Vayikra (11:43) which speaks of Tum’a with the word “Ve’nitmetem” (“you shall become defiled”). The Gemara notes that the word is spelled without the letter “Alef,” such that it may be read as referring to “Timtum” (“blockage”), thus alluding to the fact that impurity has the effect of “Timtum Ha’leb” – confusing one’s mind. The Torah therefore forbade a person in a state of impurity to enter the sacred area of the Bet Ha’mikdash, just as a person wearing unseemly clothing is not permitted to enter a royal palace. In Megilat Ester, for example, we read that it was forbidden to enter the area in front of the king’s palace while wearing sackcloth (“Ki En La’bo El Sha’ar Ha’melech Bi’lbush Sak” – 4:2). Tum’a is, in a sense, spiritual sackcloth, as it makes a person spiritually “unseemly,” and thus a person is barred from the area of the Bet Ha’mikdash when in a state of impurity.
There are three different categories of Tum’a, and each category requires distancing oneself from a different region. A person who is declared a Mesora – impure as a result of a Sara’at skin infection – is barred from the entire city of Jerusalem. People who have become impure as a result of bodily emissions (Nida, Zab, Zaba), and women after childbirth, are barred from the Temple Mount, but may enter the rest of the city of Jerusalem. One is impure as a result of contact with a human corpse is permitted even on the Tempe Mount, but must avoid the area of the Bet Ha’mikdash.
This command is binding upon both males and females, and at all times, even nowadays, as the site of the Bet Ha’mikdash retains its sacred status even after its destruction. As we are all considered Tameh (impure), it is forbidden even today to enter the area where the Bet Ha’mikdash stood.
An impure individual who enters an area from which he is barred due to his state of Tum’a has transgressed this affirmative command, as well as a Biblical prohibition, which is listed as a separate Misva.