Leviticus 13-14 contains lengthy description of some sort of scaly disease that can inflict a person's skin, clothes or a home. Our tractate is an extended commentary, explanation and clarification of these laws. This is not the place for a critical commentary on these laws, why they occupy such an important place in the Torah, their parallels in other ancient cultures, and other such topics. For that I refer the reader to the JPS Commentary on Leviticus, or for more detailed reading, to the Anchor Bible Commentary on Leviticus. I strongly suggest reading these chapters carefully before beginning to learn the mishnaic material.
A note on my translation: many scholars doubt whether the Torah refers to the disease now known as leprosy. In my translation I have tried to use the transliterated terms as much as possible. This way the description the rabbis provide will speak for itself, rather than knowledge from the outside determining the meaning of these terms. Thus I will use the word "nega" or "negaim" to mean some sort of skin, clothing or house infliction. The Torah and the rabbis frequently use the word "baheret" or "se'et" to refer to these "plagues." I will occasionally use these terms as well, but "baheret" will usually be translated as white spot. This will, I hope, increase our familiarity with the Hebrew original as well.
Let me just give a brief intro to how the "nega" process works. A nega is a white spot found on a person's skin. In order for the nega to be impure it must have one of three signs: 1) white hairs; 2) quick flesh, which means that there is some live flesh within the nega; 3) the spreading of the nega. If the nega doesn't have one of these signs, the person is isolated for a week. If after the first week he still doesn't have one of these signs he is isolated again. If after two weeks he doesn't have one of the signs, he is pronounced pure. If he is declared impure at any time, he must burn his clothes and he is sent away from the camp.
The mishnah also talks about other skin problems, namely the boil (shekhin) and the burn (mikhvah). It also discusses negaim that occur on the head or beard, for they have their own rules and regulations.
For the rabbis, since there are so many verses in the Torah on the topic, it is one that will quite obviously occupy them (and there are fourteen chapters in this tractate). What I am trying to say (in a bit of a roundabout manner) is that even if negaim (scale disease) was not familiar to them, and was of no practical concern, it is "real" to them because the Torah pays so much attention to it. Chapter 13 1 The Lord spoke to Moses and Aaron, saying: 2 When a person has on the skin of his body a swelling, a rash, or a discoloration, and it develops into a scaly affection on the skin of his body, it shall be reported to Aaron the priest or to one of his sons, the priests. 3 The priest shall examine the affection on the skin of his body: if hair in the affected patch has turned white and the affection appears to be deeper than the skin of his body, it is a leprous affection; when the priest sees it, he shall pronounce him unclean. 4 But if it is a white discoloration on the skin of his body which does not appear to be deeper than the skin and the hair in it has not turned white, the priest shall isolate the affected person for seven days. 5 On the seventh day the priest shall examine him, and if the affection has remained unchanged in color and the disease has not spread on the skin, the priest shall isolate him for another seven days. 6 On the seventh day the priest shall examine him again: if the affection has faded and has not spread on the skin, the priest shall pronounce him clean. It is a rash; he shall wash his clothes, and he shall be clean. 7 But if the rash should spread on the skin after he has presented himself to the priest and been pronounced clean, he shall present himself again to the priest. 8 And if the priest sees that the rash has spread on the skin, the priest shall pronounce him unclean; it is leprosy. 9 When a person has a scaly affection, it shall be reported to the priest. 10 If the priest finds on the skin a white swelling which has turned some hair white, with a patch of undiscolored flesh in the swelling, 11 it is chronic leprosy on the skin of his body, and the priest shall pronounce him unclean; he need not isolate him, for he is unclean. 12 If the eruption spreads out over the skin so that it covers all the skin of the affected person from head to foot, wherever the priest can see — 13 if the priest sees that the eruption has covered the whole body — he shall pronounce the affected person clean; he is clean, for he has turned all white. 14 But as soon as undiscolored flesh appears in it, he shall be unclean; 15 when the priest sees the undiscolored flesh, he shall pronounce him unclean. The undiscolored flesh is unclean; it is leprosy. 16 But if the undiscolored flesh again turns white, he shall come to the priest, 17 and the priest shall examine him: if the affection has turned white, the priest shall pronounce the affected person clean; he is clean. 18 When an inflammation appears on the skin of one's body and it heals, 19 and a white swelling or a white discoloration streaked with red develops where the inflammation was, he shall present himself to the priest. 20 If the priest finds that it appears lower than the rest of the skin and that the hair in it has turned white, the priest shall pronounce him unclean; it is a leprous affection that has broken out in the inflammation. 21 But if the priest finds that there is no white hair in it and it is not lower than the rest of the skin, and it is faded, the priest shall isolate him for seven days. 22 If it should spread in the skin, the priest shall pronounce him unclean; it is an affection. 23 But if the discoloration remains stationary, not having spread, it is the scar of the inflammation; the priest shall pronounce him clean. 24 When the skin of one's body sustains a burn by fire, and the patch from the burn is a discoloration, either white streaked with red, or white, 25 the priest shall examine it. If some hair has turned white in the discoloration, which itself appears to go deeper than the skin, it is leprosy that has broken out in the burn. The priest shall pronounce him unclean; it is a leprous affection. 26 But if the priest finds that there is no white hair in the discoloration, and that it is not lower than the rest of the skin, and it is faded, the priest shall isolate him for seven days. 27 On the seventh day the priest shall examine him: if it has spread in the skin, the priest shall pronounce him unclean; it is a leprous affection. 28 But if the discoloration has remained stationary, not having spread on the skin, and it is faded, it is the swelling from the burn. The priest shall pronounce him clean, for it is the scar of the burn. 29 If a man or a woman has an affection on the head or in the beard, 30 the priest shall examine the affection. If it appears to go deeper than the skin and there is thin yellow hair in it, the priest shall pronounce him unclean; it is a scall, a scaly eruption in the hair or beard. 31 But if the priest finds that the scall affection does not appear to go deeper than the skin, yet there is no black hair in it, the priest shall isolate the person with the scall affection for seven days. 32 On the seventh day the priest shall examine the affection. If the scall has not spread and no yellow hair has appeared in it, and the scall does not appear to go deeper than the skin, 33 the person with the scall shall shave himself, but without shaving the scall; the priest shall isolate him for another seven days. 34 On the seventh day the priest shall examine the scall. If the scall has not spread on the skin, and does not appear to go deeper than the skin, the priest shall pronounce him clean; he shall wash his clothes, and he shall be clean. 35 If, however, the scall should spread on the skin after he has been pronounced clean, 36 the priest shall examine him. If the scall has spread on the skin, the priest need not look for yellow hair: he is unclean. 37 But if the scall has remained unchanged in color, and black hair has grown in it, the scall is healed; he is clean. The priest shall pronounce him clean. 38 If a man or a woman has the skin of the body streaked with white discolorations, 39 and the priest sees that the discolorations on the skin of the body are of a dull white, it is a tetter broken out on the skin; he is clean. 40 If a man loses the hair of his head and becomes bald, he is clean. 41 If he loses the hair on the front part of his head and becomes bald at the forehead, he is clean. 42 But if a white affection streaked with red appears on the bald part in the front or at the back of the head, it is a scaly eruption that is spreading over the bald part in the front or at the back of the head. 43 The priest shall examine him: if the swollen affection on the bald part in the front or at the back of his head is white streaked with red, like the leprosy of body skin in appearance, 44 the man is leprous; he is unclean. The priest shall pronounce him unclean; he has the affection on his head. 45 As for the person with a leprous affection, his clothes shall be rent, his head shall be left bare, and he shall cover over his upper lip; and he shall call out, "Unclean! Unclean!" 46 He shall be unclean as long as the disease is on him. Being unclean, he shall dwell apart; his dwelling shall be outside the camp. 47 When an eruptive affection occurs in a cloth of wool or linen fabric, 48 in the warp or in the woof of the linen or the wool, or in a skin or in anything made of skin; 49 if the affection in the cloth or the skin, in the warp or the woof, or in any article of skin, is streaky green or red, it is an eruptive affection. It shall be shown to the priest; 50 and the priest, after examining the affection, shall isolate the affected article for seven days. 51 On the seventh day he shall examine the affection: if the affection has spread in the cloth — whether in the warp or the woof, or in the skin, for whatever purpose the skin may be used — the affection is a malignant eruption; it is unclean. 52 The cloth — whether warp or woof in wool or linen, or any article of skin — in which the affection is found, shall be burned, for it is a malignant eruption; it shall be consumed in fire. 53 But if the priest sees that the affection in the cloth — whether in warp or in woof, or in any article of skin — has not spread, 54 the priest shall order the affected article washed, and he shall isolate it for another seven days. 55 And if, after the affected article has been washed, the priest sees that the affection has not changed color and that it has not spread, it is unclean. It shall be consumed in fire; it is a fret, whether on its inner side or on its outer side. 56 But if the priest sees that the affected part, after it has been washed, is faded, he shall tear it out from the cloth or skin, whether in the warp or in the woof; 57 and if it occurs again in the cloth — whether in warp or in woof — or in any article of skin, it is a wild growth; the affected article shall be consumed in fire. 58 If, however, the affection disappears from the cloth — warp or woof — or from any article of skin that has been washed, it shall be washed again, and it shall be clean. 59 Such is the procedure for eruptive affections of cloth, woolen or linen, in warp or in woof, or of any article of skin, for pronouncing it clean or unclean.