ויקרא פרק יד לה ּובָא אֲשֶׁ ר-לֹו הַבַיִת וְ הִ גִ יד לַכֹּהֵן לֵאמֹּר כ ְּנֶׁגַע נִרְ אָ ה לִי בַבָיִת
Rashi explains that a person should not say that he has seen something which is definitely a plague in the house, but should only say ‘like a plague’. The source for Rashi is a Mishna in Negaim chapter 12 (Mishna 5). The commentaries there try to explain the reason for saying ‘like’ instead of definitely. Look at the Tosfos YomTov there (who brings many different explanations).
Perhaps, we can explain the reason as follows. If the person would say that it is definitely a plague, he would be deciding the Halacha, that the house is certainly impure. Really this is a task that is given to the Cohen, to decide the status of the house. Sometimes there is a discoloration that looks like a plague which it is not, and it is the Cohen who is the expert in this. If the house owner decides the Halacha he is transgressing the prohibition of making a Halachic ruling in front of his teacher, and there is a great punishment associated with this (see Eruvin 63a).
If so, we can better understand what the Rabbis taught in Toras Cohanim. It states there that the Cohen should rebuke the owner of the house, and say, ‘my son, you should know that the plague only comes onto a house because of speaking lashon hara’. Rabbi Shimon ben Elazar says, ‘also because of pride’.
It is not clear why they chose only those two reasons for the plague, when the Gemara in Erechin (16a) gives seven different reasons – what is special about these two?
We can explain based on the Talmud there (Erechin 16a). It was taught: Rabbi Elazar ben Parta asks, “from where do we see how great is the sin of lashon hara? They answer is from the spies (that Moshe sent to Israel). They only spoke badly about sticks and stones (as the Torah tells us in Shelach Lecha – “They spoke bad things about the land”). If the punishment is so great for speaking about inanimate objects, how much more so will a person be punished for speaking lashon hara about another person!
We see from here that there is even a prohibition of speaking lashon hara against sticks and stones. This explains how the Cohen must rebuke the home owner. If the owner says ‘I have seen a plague in my house’ (meaning definitely) he is in effect saying lashon hara against his house (since he is not deciding the Halacha or saying it for any constructive purpose), which is forbidden even on sticks and stones, and even on a house. Therefore he must say ‘like a plague’ which is not saying something definitely bad about the building (and which is for the purpose of informing the Cohen that he must come to decide the Halacha).
Rabbi Shimon ben Elazar comes to add that the house owner must not make a definite decision about the matter, because that is the task of the Cohen. If the house owner makes the Halachic ruling in front of the Cohen that is a sign of excessive pride, which is one of the reasons that the plague came in the first place.
Therefore the Toras Cohanim didn’t tell the Cohen to list all the other reasons for the plague, because they don’t have a connection to the difference between saying ‘a plague’ in lieu of ‘like a plague’.
ויקרא פרק יד לג וַיְדַ בֵר ה' אֶׁ ל-משֶׁ ה וְ אֶׁ ל- אַ הֲרֹּן לֵאמֹּר: לד כִ י תָ בֹּאּו אֶׁ ל-אֶׁ רֶׁ ץ כְ נַעַן אֲשֶׁ ר אֲנִי נֹּתֵ ן לָכֶׁם לַאֲחֻזָה וְ נָתַ תִ י נֶׁגַע צָרַ עַת בְ בֵית אֶׁ רֶׁ ץ אֲחֻזַתְ כֶׁם
The Gemara (Sanhedrin 71a) says Tzara’as of a house never happened or will happen. It is only in the Torah to expound and receive reward. Rabbi Epstein says this is alluded to in the verse that was told to Moshe and Aharon. Thus, just like Moshe & Aharon never went into Eretz Yisrael, so Tzara’as by a house also did not happen. Most Rabbis disagree with this statement and say Tzara’as of the house did occur.