Zavim, Chapter 2, Mishnah 1. Having described in the opening chapter how the sightings of a zav are counted, the Mishnah now turns to a different question altogether: which people are capable of becoming tamei through zivah in the first place? Not every category of person is automatically included in every law of tumah, so the Mishnah lays out a list.
The Mishnah opens with a sweeping statement: "Hakol mitam'in b'zivah", everyone is subject to contracting and conveying this tumah. The Gemara in Erchin adds how far this reaches: even an infant one day old is included in this halachah. From there the Mishnah begins to spell out the individuals one might otherwise have excluded.
Gerim and Avadim
"Af hageirim", converts are included, and "af ha'avadim", the eved Kena'ani as well, "bein meshuchrarim bein she'einan meshuchrarim", whether they have been set free or are still in servitude.
Why would we have thought otherwise? Because the pasuk introduces the subject with the words "Dabru el bnei Yisrael", speak to the children of Israel. From that phrasing our Sages derived a limitation: "bnei Yisrael mitam'in b'zivah v'ein hagoyim mitam'in b'zivah", a Jew is subject to the tumah of zivah while a non-Jew is not. Given such a restriction, one might reasonably have assumed that the halachah applies only to those born into Klal Yisrael. The Toras Kohanim, the Midrash Halachah on Sefer Vayikra, teaches otherwise. The verse continues with the words "va'amartem aleihem", and you shall say to them, and that additional expression comes to bring in the geirim and the avadim. It is precisely because of this derashah that our Mishnah is able to state plainly that converts and slaves are included.
Those Lacking Full Standing or Full Definition
The Mishnah continues its list. A cheresh, a deaf-mute, and a shoteh, one who lacks the mental and intellectual capacity to be held responsible, are both subject to this tumah, even though in many areas of halachah their standing is diminished. So too a katan, a minor.
Also included is a seris adam, a man whose reproductive organ was removed or damaged through human intervention, and a seris chamah, one who was naturally never capable from birth. Whether the condition came about through surgery or through nature, both are subject to the halachos of zivah.
Tumtum and Androginos
Finally the Mishnah addresses two cases of uncertain gender: a tumtum, whose gender cannot be identified at all, and an androginos, who bears the physical characteristics of both a male and a female. Regarding these two the Mishnah rules: "nosnin aleihem chumrei ha'ish v'chumrei ha'ishah", we impose upon them the stringencies that apply to a man and the stringencies that apply to a woman. Since we either cannot determine which they are, or they present as both, every strictness of both sides is applied.
What does that mean practically? The Mishnah explains: "mitam'in b'dam ka'ishah, uv'lavan ka'ish, v'tum'asan b'safek". If such a person discharges uterine blood, that discharge conveys tumah just as a woman's flow of blood does. If such a person discharges the white flux, that conveys tumah just as a man's zivah does. But "v'tum'asan b'safek", in each case the tumah remains only a doubtful tumah. Why? Because with a discharge of blood, it is entirely possible that this individual is halachically a male, in which case such a discharge would leave him tahor. And with a discharge of flux, it is possible that this individual is halachically a female, in which case that flow would not render her tamei. Each emission on its own therefore leaves us in a state of safek.
If, however, this person experienced both a discharge of blood and a discharge of flux, the tumah is no longer in doubt at all, and terumah that came into contact with them is burned. The logic is straightforward: whichever of the two this person actually is, one of those two emissions definitely renders them tamei. There is no longer any escape from the conclusion.
The Makom HaMikdash
There is nevertheless a striking exception. Even with both emissions, such a person is not liable for entering the makom hamikdash, the sanctified area of Har Habayis, while in that state of tumah. The source is the pasuk "mizachar ad nekeivah teshaleichu", from male to female you shall send out. The Torah speaks of sending out a definite male and a definite female. A tumtum or an androginos, whose gender status is by definition unresolved, is not covered by that expression. Consequently, while their tumah itself is real enough to require the burning of terumah, they do not incur liability for ascending Har Habayis in that state.
The Mishnah has thus drawn a broad circle: infants, converts, slaves free and unfreed, the cheresh, the shoteh, the katan, the seris of either kind, and even those whose very gender cannot be established. Virtually no Jew stands outside the halachos of zivah, and where identity is uncertain, the stringencies of both sides are applied, except where the Torah's own wording demands a definite male or female.