Certain things don’t translate well. The English word for tefillin is phylacteries, but that’s arguably even less helpful than saying tefillin. Kosher means fit (to eat), but we all know that doesn’t even scratch the surface of what it means to be kosher. Even words such as “Sabbath” and “prayer” fail to capture the nuances of “Shabbos” and “tefillah.” But if there were ever words that don’t translate, they’re “metzora” and “tzara’as.”
We may call the metzora a “leper” and tzara’as “leprosy,” but those terms are more than inadequate; they’re downright inaccurate. Tzara’as was not leprosy. It was a skin affliction to be sure, but it had a unique manifestation and it was brought about as a spiritual consequence of lashon hara (gossip and slander).
This mitzvah required anyone exhibiting signs that could be tzara’as to go to a kohein (priest), who was trained in identifying this affliction. (The fact that he went to a kohein instead of to a doctor is our first clue that this is not a simple medical condition.) He must then do all that the kohein instructs him in this matter.
Tzara’as would occur when one’s skin developed a white patch or patches. A kohein was trained to identify a variety of shades of white in order to be able to diagnose tzara’as. (A non-kohein could also learn to tell the difference between snow-white, eggshell-white, etc. but only a kohein could declare someone clean or unclean. This is a classic example of some mitzvos being given to some people and some mitzvos being given to others. See Mishna Negaim 3:1.) The person with the symptoms might be quarantined for one or two seven-day periods in order to see if the afflicted area changes. The kohein would declare the person clean or unclean based on the results.
The Talmud (Arachin 15b) tells us that tzara’as was brought about because of lashon hara. Homiletically, the word is related to “motzi sheim ra” – slandering another person. Tzara’as is a fitting repercussion for gossip; just as the tale-bearer’s words separated others, the tzara’as separates him. The quarantine period gives the gossip an opportunity to reflect upon his actions and change his ways. (The worst thing he could do would be to think, “This is all just a big coincidence.” The entire purpose of this mitzvah is a message from God to shape up!)
We see tzara’as as a consequence of lashon hara in the Torah itself. Most famously, when Miriam spoke ill of Moshe, she was smitten with tzara’as and quarantined outside the camp (see Numbers chapter 12). Even Moshe was struck with a mild case of tzara’as, when he predicted to God that the nation wouldn’t listen to him; the sign of tzara’as was meant as a reminder not to speak ill of the people (See Exodus 4:6 and Rashi there).
This mitzvah applied to both men and women but only when there were kohanim sufficiently trained in the matter (which is no longer the case). The details of this mitzvah are discussed in the Mishna in tractate Negaim; in the Talmud, see tractate Shevuos, 5b-6b. It is codified in the Mishneh Torah in the Hilchos Tumas Tzara’as and it is #101 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos. The Ramban (Nachmanides) does not count this among the 613 mitzvos.