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Based on this, the Kessef Mishneh (in his gloss to Chapter 5, Halachah 8) asks: What is the difference between the baldness that causes a person to be classified as a keireiach or a gibeiach and a netek? In resolution, he explains that those types of baldness are ordinary phenomena that result from a person's changing hormonal condition. The disappearance of hair from a netek, by contrast, is a departure from the norm and comes about as a result of a tzara’at affliction.
Based on this, the Kessef Mishneh (in his gloss to Chapter 5, Halachah 8) asks: What is the difference between the baldness that causes a person to be classified as a keireiach or a gibeiach and a netek? In resolution, he explains that those types of baldness are ordinary phenomena that result from a person’s changing hormonal condition. The disappearance of hair from a netek, by contrast, is a departure from the norm and comes about as a result of a tzara’at affliction.

See Chapter 1, Halachah 7. In his Commentary to the Mishnah (Nega’im 10:6), the Rambam cites the Sifra that notes that Leviticus 13:31 mentions “the blemish (nega) of the netek,” establishing an equivalence between the two. Just as a blemish must be a gris to impart impurity, so too, a netek must be a gris to impart impurity.

In contrast to blemishes on ordinary flesh that must appear “deep,” i.e., beneath the surface of the skin (Chapter 1, Halachah 6).

In the above passage from Leviticus, the term appears several times.

A man who was born without sexual potency or castrated (Hilchot Ishut 2:11-13).

See the conclusion of Chapter 5.

As there is for other blemishes. See Chapter 1, Halachah 2.

Leviticus, op. cit.

Our text follows the version found in authoritative manuscripts of the Mishneh Torah. The standard published text follows a different — and more problematic — version.

In his Commentary to the Mishnah (Nega’im 10:1), the Rambam states that this hue appears as a blend between yellow and red.

See Halachah 4.

Or other ordinary colored hair. See Rashi’s Commentary to Leviticus 13:37.

I.e., more of his hair fell out, leaving a greater area bare.

See Halachah 6.

Since ordinary hair is growing in that area, it is no longer considered as a netek.

I.e., leaving two hairs on all sides of the netek (Sifra; see the following halachah). The intent of the adjuration (Leviticus 13:33): “He shall not shave the netek” is that these hairs be left; since the netek is hairless, there is obviously no point in shaving it (Sifra). As mentioned in Chapter 10, Halachah 1, shaving these hairs is considered as a Scriptural prohibition.

As is the ruling with regard to an ordinary baheret (Chapter 1, Halachah 10).

In contrast to a shaving to achieve purity that must be performed by a priest (Chapter Halachah 1).

Note the Mishneh LeMelech who debates whether this charge should be considered as an independent mitzvah. Sefer HaMitzvot (positive commandment 101) mentions this as part of the mitzvah to rule regarding a tzara’at blemish.

In contrast to the shaving cited that must be performed with a razor (ibid.).

Who is forbidden to shave his hair.

And the priest should judge whether or not the netek spread.

Leviticus 13:30, et al.

I.e., shorter than ordinary hair; note, however, the conclusion of the following halachah. Nega’im 10:1 explains that dak means “damaged” and explains that the intent is that the hair is short..

In contrast to the white hair mentioned in Chapter 2, Halachah 6.

See Chapter 2, Halachah 1, for an explanation of this translation.

This measure is significant in several halachic contexts. See Hilchot Ishut 2:16; Hilchot Nizirut 6:1. This is a larger measure than the one mentioned in the previous halachah.

As the Rambam proceeds to explain, this criterion applies only when such hair remains after a netek erupted. Different rules apply if such hair grew within an existing netek.

For they are not considered as being part of the netek, but as being part of the hair around it.

Because the growth of such hair indicates that the netek has undergone a change.

Although a white hair is not a sign of impurity in this context, it also does not save the netek from being categorized as impure. From Rashi’s Commentary to Leviticus 13:37, some commentaries maintain that hairs of two colors other than white can prevent the person from being categorized as impure.

The Kessef Mishneh quotes Rav Yosef Corcus as explaining that this clause is referring to an instance where the netek has spread and two hairs grow, one black and one golden. Nevertheless, a golden hair — which could serve as part of a sign of impurity — cannot become a sign of purity.

I.e., less than the measure mentioned in the previous halachah.

This clause does not begin with the term “originally,” because the spreading of a netek requires an interval of a week’s isolation.

Each at least the size of a gris.

The line of hair does not prevent them from being classified as netakim, because it is at the side of each netek and not in the midst of them. See Halachah 6.

Thus the person is considered as impure (Nega’im 10:6). Here the Rambam does not use the term “impure,” leading commentaries to infer that he is speaking about a person in isolation to determine whether the netek will become impure or not. On that basis, there are those who raise a question concerning the Rambam’s ruling, for seemingly, by increasing to cover part of the line of hair, the size of the netek increased sufficiently to render the person impure. Diagram

And thus causes the person to be considered as pure, as stated there.

Otherwise, the hair is not considered to be in the center of the netek (Halachah 6).

The Ra’avad writes that this ruling applies even if the hair is broken through in two places the size of a gris. It is, however, questionable whether the Rambam would agree, for seemingly, in such an instance, the black hair would be in the center of the netek.

Thus despite the increase of the size of the netek, the person is considered as pure.

For it is considered as an extension of the existing netek(im) and the signs of impurity that appear in it render that netek impure.

For it is considered as having grown in the existing netek(im).

For the hairs are not considered in the midst of the netek, but at its side, as evident from Halachah 6. Only when the strand is a gris wideis it possible for the existing hairs to save it from impurity. Note also the parallels to Chapter 4, Halachah 7.

As long as the person does not have a place on his head with two hairs together, this leniency applies.

Note the contrast to Chapter 7, Halachah 1, with regard to a baheret that spread over a person’s entire body.

This also applies to the skin on top of a person’s head.

Chapter 5, Halachah 11; Chapter 6, Halachah 2. Diagram

This parallels the ruling that applies with regard to a baheret as stated in Chapter 7, Halachah 2. In both instances, to be considered pure when the blemish covers the entire area, the blemish must have been viewed by the priest previously.

I.e., the only sign of impurity that is relevant for such an individual.

I.e., as is the law regarding a smaller netek.

The Ra’avad states that if black hair grows, even if it later falls off and golden hair remains, the netek is pure. (The Kessef Mishneh notes that this can be inferred from Halachah 8.) The Ra’avad continues that the above applies, however, when the black hair grows after the person was released from the inspection process and then deemed pure. If the black hair grows in an initial situation, he can become impure again as stated in the following clause.
The Kessef Mishneh cites Rav Yosef Corcus who explains the difference between the two situations. When the person had already been deemed impure and then black hair grows, it is appropriate to apply the words of Leviticus 13:37: “The netek has been healed.” When, by contrast, the person was not deemed impure, it is not appropriate to speak of the existence of a netek and thus it cannot be defined as having been healed.

I.e., it spread to the area covered by the black hair. The Kessef Mishneh states that this ruling applies only when the black hair grew at its side. If it grew in the center, he is pure, because there is no concept of a blemish increasing into its own midst.

I.e., if a person’s entire head grows bald because of a netek and his beard remains, he is deemed pure. The fact that his beard remains is not relevant. Conversely, hair on his head does not affect the ruling with regard to hair falling from his beard. The two areas are viewed independently [see the Rambam’s Commentary to the Mishnah (Nega’im 10:9)].

I.e., if two netakim less than a gris each are next to each other, one on the head and one, on the area of the beard, they are not combined and considered as a netek of the minimum size.

I.e., spreading from one to the other is not considered as a sign of impurity, but as a new netek.

From the mention of the term (HEBREW_TEXT) before the term tzara’at, we derived that the two are separate categories.

See Hilchot A vodat Kochavim 12:7 where the Rambam gives a similar prohibition with regard to the prohibition against shaving.

Our translation of the latter term is based on the Rambam’s Commentary to the Mishnah (op. cit.).

Arachin 3a derives this concept from Leviticus 13:2 which states: “A man, when there will be a blemish....” “A man” implies any man, regardless of his age or state.

Since they are obligated in the observance of certain mitzvot, the laws of impurity also apply to them.

For these laws apply only to members of the Jewish people.

As stated in Hilchot Issurei Bi’ah 14:7, this term refers to “a non-Jew who makes a commitment not to worship false deities and to observe the other [six] universal laws commanded to Noah’s descendants. He does not circumcise himself or immerse [in a mikveh].” See also Hilchot Avodat Kochavim 10:6; Hilchot Melachim 8:10.

The Kessef Mishneh and others have noticed the apparent contradiction between the Rambam’s ruling here and Chapter 1, Halachah 3, where he states: “Any priest who does not recognize the different appearances [of blemishes] and their names when he is taught and informed, should not assess a blemish until he understands [them] and recognizes [them].” On the basis of Arachin 3a, the Kessef Mishneh resolves the issue, explaining that the ruling in Chapter 1 applies when a person relies on his own judgment, while here, he is willing to follow the guidance of others, as stated in Halachah 2.
Kin’at Eliyahu notes that source, but maintains that this ruling is complemented by Halachah 2. Here the Rambam is stating that any person can develop the expert knowledge to determine what is a blemish and what is not; i.e., he can serve as a resource for such a ruling. The pronouncement of that ruling, however, is dependent on a priest, as stated in Halachah 2.

The Torah does not allow him to pass judgment on his own condition. The sages of mussar (ethical development) have drawn a homily from this law. A person may be able to appreciate the personal blemishes of others, but when it comes to his own, his self-interest prevents him from being objective and seeing his faults, no matter how serious they are.

As stated in Halachah 5, a challal, a person whose priestly lineage is tainted, is also unacceptable.

I.e., he is not an expert in determining what is a blemish and what is not. Based on Arachin, op. cit., the Kessel Mishneh explains that the priest may not be totally unknowledgeable. He must be able to understand why the sage makes the ruling he does even if he is incapable of reaching it on his own.

The prooftext specifically mentions a priest.

The Kessel Mishneh cites the commentary of Rabbenu Asher to Nega’im 3:1 which explains that we are not speaking of a person who is mentally incapable in a simple sense — for he will not be able to comprehend the words of the sage (see note 8). Instead, we are speaking about a man of understanding who is referred to as mentally incapable only when compared to a Torah sage. Nevertheless, the terms the Rambam uses are never interpreted as suggested by the Kessel Mishneh.

I.e., even one that is obvious.

See Chapters 12-15. From the Rambam’s words, it appears that even if a priest is knowledgeable with regard to blemishes that affect humans, he cannot pronounce judgment upon them unless he is also familiar with the other blemishes [Rav Yosef Corcus, based on the Toselta (Nega’im 1:1)].

Significantly, the Rambam alters the order in which the verses appear in the Torah. Also, the quote from the verse is not exact. Some suggest that a printing error crept into the text.

The implication is that the priest’s declaration must correspond to the halachic reality.

I.e., he must be able to compare the condition of the blemish to that of the healthy skin next to it (Rav Yosef Corcus). See Chapter 6, Halachah 3.

In which instance, it would not be a sign of impurity (Chapter 2, Halachah 6).

A challal is the son of a priest conceived through relations forbidden to a priest or the son of a challal. None of the mitzvot of the priesthood apply to him. See Hilchot Issurei Bi’ah 19:10.

See Hilchot Bi’at HaMikdash, ch. 6, which explains the commandment forbidding a priest with a blemish from serving in the Temple and, ch. 7, which describes those disqualifying blemishes.

For his priesthood is intact.

Implied is that his vision must be intact in its totality. A priest who is color blind is unfit to make these determinations. If the priest requires glasses, it can be said that the following ruling applies. If he could make an appropriate determination without glasses while standing very close to a person and the glasses merely enable him to do so from further away, he is acceptable. If, however, he requires glasses to make the determination even when standing close, he is disqualified.

Leviticus 13:5, 6, 14, et al.

When the sun shines brightly.

I.e., on a day when the sun rises at 6 AM and sets at 6 PM, between 9 and 10 AM. In this context, an “hour” is understood as one twelfth of the time between sunrise and sunset.

From Leviticus 14:54-55, the Sifra derives an equivalence between all blemishes.

In his Commentary to the Mishnah (Nega’im 1:4), the Rambam states the rationale for this ruling: Determining whether a blemish is pure or impure resembles a legal judgment and legal judgments are not made on the Sabbath.

More specifically, the term yom tov refers to the sacred days of the festivals, excluding chol hamo’ed. Nevertheless, from Nega’im 3:2 and Mo’ed Kattan 7a, it appears that the determination of the status of blemishes should not be made on Chol HaMo’ed as well. Indeed, the Rambam himself rules this way in Hilchot Sh’vitat Yom Tov 7:16 and this conclusion can be deduced from Halachah 8. Accordingly, the term he uses here has attracted the attention of the commentaries.

On which the blemish is inspected for a second time by the priest.

The commentaries have raised a question: Since a blemish that reappears with signs of impurity is deemed impure (Chapter 1, Halachah 1 0), how is this a stringency, the person would have been deemed impure regardless? They explain that if the person had appeared on the Sabbath without signs of impurity, he would have been isolated. Thus (as stated in Halachah 9), even if signs of impurity appeared during the week, he would not be judged again for seven days and it is possible that in that time, the signs of impurity could have disappeared.

See Hilchot Ishut 10:12 which states that a man who weds a virgin should celebrate with her for seven days, refraining from pursuing his occupation. Celebratory feasts are held during these days.

In Hilchot Sh’vitat Yom Tov, loc. cit., the Rambam gives the rationale for this ruling: “His festival will be transformed into mourning.”

I.e., remove the utensils contained within.

Celebrating a marriage, a festival, or the like.

If there is a blemish of the appropriate size without a sign of impurity.

If it possesses a sign of impurity.

If at the outset, it is not of the -appropriate size or if, after a period of isolation or impurity, it covered the entire area, or if after periods of isolation, no signs of impurity appeared.

In his Commentary to the Mishnah (Nega’im 3:1), the Rambam. states that this is a Scriptural decree that is beyond our understanding.
If for some reason, an inspection was not made on these days, it may be made afterwards.

I.e., it is the seventh day of the first week and the first day of the second week.

I.e., if a person was isolated after the appearance of a tzara’at blemish and another blemish appeared during the week of isolation, the second blemish should not be examined by a priest and the person isolated because of it.

Although the sign of impurity would cause the person to be deemed impure when seen by the priest, the priest does not make his examination or deliver his ruling until the end of the week of isolation.

In his Commentary to the Mishnah (op. cit.), the Rambam cites the Sifra which sees an allusion to this concept in Leviticus 13:1: “He shall not isolate it, for he is impure.”

Although the blemish is of the type that would warrant these rulings, since the person is already definitively impure, the second blemish is not considered at the time it is discovered. Nevertheless, when the priest inspects the person for the first blemish, he also takes note of the second.

Before delivering a ruling regarding the first.

I.e., simultaneously, with the same glance. Instead, the priest must examine one, consider its status, and then examine the other.

I.e., an exclusion is implied; one, but not two. Significantly, in his Commentary to the Mishnah (op. cit.), the Rambam cites a different prooftext.

In this context, the phrase implies that the blemish must be able to be perceived by the person’s ordinary sight.

The priest need not examine the above portions of his body before deeming him pure, as stated in Chapter 6, Halachah 1.

In which instance, he is required to spread his legs.

In which instance, he is required to stretch out his arms. Nevertheless, an olive tree is mentioned, because these trees are not all that high and the person need not strain when reaching up.

The bracketed addition is based on the Rambam’s Commentary to the Mishnah (Nega’im 2:4).

In which instance, she spreads her legs some. She is not required to stand like a man who is hoeing, because this is not a natural and common position for a woman.

Lest there be a blemish on her breast.

Where she must stand and raise her arms, in contrast to a woman who sits and weaves with a needle.

The commentaries infer that according to the Rambam, the left underarm need not be inspected. This is also evident from his Commentary to the Mishnah (op. cit.).

It is considered as impurity of the hidden places and ignored.

As related in Chapter 1 1, Halachah 1, as part of the purification process of a person who was afflicted with tzara’at, all the hair on his or her body must be removed. Nevertheless, when inspecting the person’s body to see whether hair remains, a priest need not look in these areas, and, after the fact, hair that remains in these places does not disqualify the purification process.

Chapter 11, Halachah 1.

E. g., white hair or healthy flesh in a baheret [the Rambam’s Commentary to the Mishnah (Nega’im 7:4)].

Healthy flesh in a baheret is a sign of impurity. If, however, the healthy flesh is burned, it is no longer considered as healthy and hence, does not serve as such a sign.

I.e., not only the signs of impurity, but the entire blemish.

Sefer HaMitzvot (negative commandment 308) and Sefer HaChinuch (mitzvah 584) count this charge as one of the 613 mitzvot of the Torah.

As stated in Eruvin 96a, Makkot 13b, et al, whenever the Torah uses the term hishamar, translated as “Be... careful,” a negative commandment is involved.

As is the minimum punishment for the violation of a negative commandment.

Two white hairs are sufficient to render him impure (Chapter 2, Halachah 1).

Which is sufficient to render him impure (Chapter 3, Halachah 1).

Lashes mandated by Rabbinic decree which are given as punishment for the violation of Rabbinic commandments and for other purposes. See Hilchot Sanhedrin 16:3, 18:5, which mentions this punishment.

The bracketed addition is evident from Chapter 8, Halachot 2-3.

Sefer HaMitzvot (negative commandment 307) and Sefer HaChinuch (mitzvah 170) consider this as a separate negative commandment and counted as one of the 613 mitzvot.

If, however, he removes the hair with another instrument, he is not liable for lashes. The Ra’avad objects to this leniency.

I.e., although the signs of impurity were removed as a result of his actions, he is not liable, because he did not perform the act of removal. His actions were merely an indirect cause.

For at the time of the inspection, he did not have any signs of impurity.

As stated in Chapter 9, Halachah 9, when a person is isolated because of a tzara’at affliction, a judgment is only rendered on the seventh and thirteenth days.

In this instance as well, at the time the priest inspects him, he will not have any signs of impurity, nor had he manifested them beforehand. As Nega’im 7:4 relates, this question was posed by Rabbi Akiva to Rabban Gamliel and Rabbi Yehoshua. They did not know how to answer him. On the basis of logical deduction, Rabbi Akiva proved that the person should be considered as pure. The Rambam cites the Tosefta (Nega’im 3:4; apparently his version was slightly different from the standard published text) as stating that the other Sages accepted Rabbi Akiva’s view.

Because at present, he does not have a baheret with signs of impurity and he did not remove the baheret intentionally.

This is a stringency imposed by Rabbinic decree. The rationale is that even if the tzara’at would cover the person's entire body, one might argue that it would not have affected the flesh immediately next to it [Kessef Mishneh, based on the Tosefta (Nega’im 3:5)].

Leaving the flesh around the baheret.

Because even if the person had not pulled the hair off, there would not have been two white hairs on his baheret.

Because it was his deeds that removed the signs of impurity. The one remaining white hair could not serve as a sign of impurity alone. We do not say that since the one white hair fell off naturally, the other would also have fallen off naturally.

Here as well, the sign of impurity would have disappeared independent of the person’s actions.

Also in this instance, enough of the baheret to render the person impure remained — and disappeared naturally — after the person’s action.

Because the remaining portion of healthy skin was not large enough to render the person impure. Thus it was the person’s actions that removed the signs of impurity.

And thus the positive mitzvah of removing the foreskin will conflict with the prohibition against cutting off a baheret.

According to law, a circumcision must be performed on the eighth day of a baby's life. If, due to circumstances or even if intentionally, the circumcision was not performed on that day, it must be performed afterwards. Even so, the act is considered as a positive commandment and supersedes the prohibition against removing a baheret.

This is a general principle applicable in other instances as well. See Hilchot Nizirut 7:15, Hilchot Yibbum 6:11, Hilchot Shechitah 13:19, et al. See also the following halachah.

I.e., he becomes pure. He is required to undergo the purification process to be described in Chapter 11 and bring the sacrifices required of one who is purified from tzara’at as described in Hilchot Mechusrei Kapparah 1:3.

Sefer HaMitzvot (positive commandment 112) and Sefer HaChinuch (mitzvah 171) count this charge as one of the 613 mitzvot of the Torah.

The laws applying to a woman afflicted by tzara’at are mentioned in Halachah 8.

The Rambam is employing the interpretation of the Hebrew termperu’a suggested by Rabbi Akiva in Mo’ed Kattan 15a. However, others interpret that term as meaning to let one’s hair grow unkempt. Indeed, in Sefer HaMitzvot (op. cit., see also negative commandment 163), the Rambam follows that interpretation.

See Hilchot Evel 5:19.

The verse mentions all the acts mentioned by the Rambam.

As stated in the previous halachah. According to the Rambam’s ruling here, the negative commandment involved is the commandment for a priest not to have tom garments [see Hilchot K’lei HaMikdash 5:6; Hilchot Bi’at HaMikdash 1:10, Sefer HaMitzvot (negative commandment 164)]. As mentioned in those sources, there is a more severe dimension of this commandment relevant to the High Priest than to others and he is forbidden to have tom priestly garments.
As mentioned, there are others who interpret the term peru’a as leaving one’s hair unkempt. That is also forbidden to a High Priest (ibid.). According to these views, that prohibition is also superseded by the obligation mentioned here.

See Hilchot Eve! 5:20.

The Rambam is alluding to the three areas of Torah study: mikra (Scripture), Mishnah (the summary of Torah Law), and talmud (the in-depth study of Torah Law). See Hilchot Talmud Torah 1:11. Not only may this person study individually, he may teach others as well. See a parallel ruling regarding a person under a ban of ostracism (Hilchot Talmud Torah 7:4). A mourner, by contrast, is forbidden to study Torah (Hilchot Evel, Loc. cit.:1).

As mourners are forbidden (ibid.).

Even though there is a Rabbinic commandment to honor these days by washing and wearing special clean clothes in preparation (Hilchot Shabbat 30:2-3).

There is a difference of opinion among the Sages (Mo’ed Kattan 7b) whether a person afflicted with tzara’at is permitted to engage in physical intimacy. However, other sources — including Nega’im 14:2 — indicate that the act is permitted and the Rambam favors their ruling (Kessef Mishneh). Support for the lenient view is drawn from the fact that King Yotam was conceived by his father, Uziyahu, during the time Uziyahu was afflicted with tzara’at. Uziyahu’s conduct can be used as a standard, because, although he sinned in desiring the High Priesthood, in general, he was a righteous man.

Even though these latter activities are forbidden to mourners (Hilchot Evel, op. cit.). The Kessef Mishneh explains that Mo’ed Kattan 15b leaves the question whether or not these activities are forbidden unresolved. Therefore, the Rambam rules leniently. On that basis, the Kessef Mishneh questions why the Rambam does not mention that a person afflicted with tzara’at may perform work (although a mourner may not) for this question is also left unresolved in the above source.

See Halachah 12. See also Rashi’s commentary to the prooftext which states that tzara’at comes as a punishment for malicious gossip. Since he caused separation between others, he is forced to live alone.

See Hilchot Beit HaBechirah 7:13 which mentions this as one of the factors that cause the walled cities in Eretz Yisrael to be considered as holier than the remainder of the land.

Sotah 23b derives this from Leviticus 13:44 which speaks of “a man afflicted by tzara’at.” That can be understood as an exclusion: a man and not a woman. Note Sefer HaMitzvot (positive commandment 112) speaks of an afflicted woman cloaking her face.

A person whose genitalia are masked by a mound of flesh and it is impossible to detect his gender. See Hilchot Ishut 2:25.

A person who has both male and female sexual organs (ibid.:24).

Hence since the fulfillment of a Scriptural commandment is involved, they must adopt the stringency (Kessef Mishneh).

The obligation for a person deemed impure because of tzara’at to shave his body and bring two fowl is described at the beginning of the following chapter.

Nor must he cover his head or rend his garments. Tosafot Yom Tov adds that in isolation, a person need not cover his face. Moreover, a person who was isolated for tzara’at is not obligated to bring the sacrifices mentioned in Hilchot Mechusrei Kapparah, ch. 4, as is one who was deemed definitively impure (Kessef Mishneh).

See the definition of this term in Hilchot Tuma’at Meit 5:7.

See Hilchot Tuma’at Meit 1:5, 6:3; Hilchot Keilim 13:1-2.

Even when the person carrying him never touched him, e.g., the impure person sat on a chair and another person carried the chair without touching him.

Which itself does not contract ritual impurity.

And thus he never touched the object on which he sat or lay.

A man who has three successive emissions from his sexual organ similar to those resulting from a gonorrheal infection. (See Hilchot Mechusrei Kaparah 2:1-2).

A woman who experienced uterine bleeding for three consecutive days at a time other than the days when she usually menstruates (ibid. 1:6).
With regard to the concept that such individuals impart impurity to the objects over which he lies or sits even if there is a substance intervening between them, see Hilchot Metamei Mishkav UMoshav 6:5.

I.e., immerse in a mikveh.

As recorded in the Sifra to that verse.

See Hilchot Metamei Mishkav UMoshav, ch. 7, for more details regarding how impurity is imparted in this manner.

For the tree is considered an ohel (“tent” or “shelter”). Since the person afflicted by tzara’at is standing there, it is considered as his “dwelling” and anyone or anything that is under it at the same time, even when he or it merely pass through, contracts impurity.

Since the afflicted person only passed under the tree and did not stand there, it is not considered as his "dwelling." Hence, he does not impart impurity.

The Kessef Mishneh gives two rationales for this ruling: a) Nega’im 5:1 states: “All doubtful situations regarding blemishes are pure;” b) since the tree is located in the public domain, the principle “any doubtful situation in the public domain is pure” can be applied.

When a person inserts his head and the majority of his body into a structure, it is considered as if he inserted his entire body [the Rambam’s Commentary to the Mishnah (Nega’im 13:8)).

A partition of this height, encompassing this area, creates a separate domain for him, even though he is visible to the others praying there [the Rambam’s Commentary to the Mishnah (Nega’im 13:8)).

By touching them, alternatively, by pausing to stand and causing the structure to become his dwelling (ibid.:12). Because of the first rationale, this restriction should also be applied to other impure people [ibid. (Zavim 3:2)].