Torah Portion: Metzora
“איש איש כי יהיה זב מבשרו…” (ויקרא טו ב)
It is a positive commandment that a zav is contaminated and he contaminates others, as it is written, Any man who will have a zav-discharge from his flesh, his discharge is contaminated. Thus shall be his contamination etc. (Vayikra 15:2-3). Scripture prescribed that anyone who experienced such a discharge shall be contaminated and can go on contaminating others until he becomes purified.
Among the roots of the mitzvah is the idea that Hashem, may He be blessed, ordained that we keep our distance from luxuries, and He commanded us that we be holy and upright in matters of food and drink, as well as in all human affairs. And without a doubt the occurrence of discharges happens upon a person when he neglects the proper use of his body. By avoiding this, we acquire for our souls the trait of being upright and balanced with all our intellect and vigor.
This matter, that one who experienced a zav-discharge is deemed tamei (contaminated), applies in all places and at all times. However, nowadays, because of our sins, we ought not reckon the contamination of zav as an applicable mitzvah because there is nothing we can do about it, whereas in the time when the Holy Temple stood a zav was required—in certain situations—to bring an offering.
One who transgressed this and entered the Temple or ate sanctified food before being purified, violated a positive commandment. This is in addition to the punishment that he is due to carry, as clarified above (in Mitzvah #167).