We previously discussed, in Mitzvos #169-177, the laws of tzara’as, commonly translated as "leprosy" but actually something else altogether. Tzara’as must be shown to a kohein in order to be evaluated and, if necessary, quarantined. Here, we are warned to be careful about tzara’as, specifically not to do anything to remove or conceal it. (The Hebrew word “hishamer,” “take heed,” denotes a negative mitzvah in that it's a warning not to do something - see Talmud Eiruvin 96a.) This mitzvah applies to all kinds of tzara’as: on people, on garments and on houses.
The reason for this is what we said about tzaraas, way back in Mitzvah #169, that tzara’as is a spiritual affliction, sent to encourage a person to change his ways. Trying to conceal it instead of dealing with it is counter-productive and shows just how set in his bad ways a person is.
An exception to this mitzvah is if an uncircumcised man has a sign of tzaraas on his foreskin. In such a case, he need not let it stop him from getting circumcised. The positive mitzvah of bris milah outranks this negative commandment.
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in tractate Shabbos (132a-b) and is codified in the Mishneh Torah in the first chapter of Hilchos Tumas Tzara’as. This mitzvah is #308 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.