The most severe type of tumah that exists is the corpse of a dead person, known as an 'avi avos hatumah' - the father of the fathers of tumah, and nothing is considered more tamei than it. Nevertheless, there are certain stringencies that apply to forms of tumah coming from other sources, which do not apply to a corpse.
The five people who are tamei - 'a zav and his companions':
A zav - a man who experienced a discharge of certain substances from his male organ over at least two consecutive days.
A zavah - a woman who experienced a flow of blood from her womb during the eleven day period following her having been a niddah for a week.
A niddah - a woman who has a flow of blood from her womb not during those eleven days.
A yoledes - a woman who has given birth.
A metzora - a man or woman afflicted with tzaraas, a particular skin disease that generally comes as a punishment for sin, and consequently it renders the person tamei.
These five types of people - even though one could have counted the zav and the zavah, the male and the female, as a single category - all fall into one category as far as their status of tumah is concerned, and they are known as "zav vachaveirav" - the zav and his companions who share the same category with him. Therefore, although we will speak below about the zav, the same law applies to the others listed as well.
Tumas midras:
One feature common to all these types of tumah is the transmission of tumas midras: when the zav or one of the others sits or lies on something whose purpose is to support a person's weight - such as a bed, which is meant for a person to lie on, or a chair - that object becomes tamei to the same degree as the zav himself. The zav has the status of an av hatumah, a high, primary level, and so do the chair and the bed. What is more: a tahor person who afterward lies on that bed becomes tamei, even if he never touched it at all. This is a unique concept of tumas midras.
Even mesama:
Another stringency that applies specifically to a zav and his companions is known as 'even mesama'. An even mesama is a very large, heavy rock that is not meant to be moved, and is virtually fixed in its place because of its weight. As a rule, many things become tamei when a person carries them, but only when he can be viewed as actually carrying the object. For example, a neveilah: one who carries it becomes tamei even without touching it. However, if the heavy rock is being held up by something, and a piece of a neveilah is resting on top of it, and a person places his hand underneath the rock - clearly he is not carrying the object that is on top of the rock.
With regard to a zav and those on his level, though, we treat it as though he is carrying the person: if the zav is on top of the rock and another person places his hand beneath it, as though carrying the rock and what is on it - that person becomes tamei. So too, if a bed was lying directly under that very rock, with no airspace between them, and the zav lay on top of the rock - we treat it as though he lay on the bed itself. Even though the person's weight is nullified and completely insignificant next to the heavy rock that is resting there in any case, in this category of people we consider his weight as being supported by the bed, and the bed becomes an av hatumah, such that one who touches it or lies on it afterward becomes tamei.
The words of the Mishnah - tumah after death:
The Mishnah says that the five people we have listed convey tumah after their death through carrying. 'Masa' generally refers to transmission of tumah that comes about through carrying, but the intent here is not the ordinary tumas masa that applies to any corpse; rather it refers to something that applies specifically to one of these five after his death: that same tumah we described called even mesama, in which we disregard the fact that the heavy stone renders the weight insignificant, and instead consider it significant. And the novelty of the Mishnah is that this law applies not only during the lifetime of the zav, but even after his death, until his flesh decomposes and disintegrates, at which point this stringency no longer applies. This does not apply to an ordinary corpse, since it is a stringency unique to these five.
It is worth knowing that this law is only rabbinic, since once a person has died he no longer has the status of a zav or a zavah, and all these forms of tumah apply specifically to someone who is still alive and in that condition. Nevertheless the Sages decreed this out of concern that people would confuse a zav or one of his companions who fainted with a zav who actually died. Therefore, as long as the matter is not completely clear - that is, as long as the body has not disintegrated and has not begun to decompose, and one could still mistakenly think that this is a zav who fainted - that same tumah remains upon him as though he were still alive.
Since this is a rabbinic decree, a non-Jew who died is not included in it. Even though a non-Jew is considered a zav, that too is only rabbinic, and we do not impose one rabbinic decree on top of another. Therefore the matter is limited to the time that the non-Jew is alive, and once he dies we no longer treat him as a zav, and so "tahor mi'letamei b'masa" - he is pure with regard to this tumah of even mesama.
The decree of "all women die as nidos":
The second half of the Mishnah deals with a further rabbinic decree. Beis Shammai say: "Kol hanashim meisos nidos" - every woman who dies is regarded as though she were a nidah. For a woman who died while she was a nidah - any garment she touched while she was a nidah required purification in a mikvah, since it became tamei through her touch. Now, every woman who dies is viewed as a nidah, and consequently any garment she touched close to her death requires purification as well.
The reason for the decree: women who were nidos in their lifetime saw that when a woman died while she was a nidah, care was taken, as part of the preparations for burial, to purify her garments as well, and this became well known and obvious. The result was that there was a noticeable distinction between a woman who died as a nidah and a woman who was not a nidah, and living women who were nidos felt a kind of embarrassment because of this open distinction. For this reason the Sages decreed that for every woman we purify the things she touched even before she died. Obviously, whatever touched her after her death became tamei in any case simply because she is a dead body; the point is that even with things she touched earlier, while she was still alive close to her death, one should act this way, in order to erase the distinction between a woman who was a nidah and one who was not, so that living women who are nidos will not be embarrassed.
Rabbi Akiva and Rabbi Yehudah hold that this concern does not exist at all, and therefore "ein nidah ela shemeisah nidah" - only a woman who died while she was a nidah is considered a nidah, and we purify the garments she touched as long as she was a nidah. In fact, Beis Hillel too disagree with the concern of Beis Shammai. Some understand that Beis Hillel do not disagree with the entire decree that Beis Shammai spoke of, but we will understand the Mishnah according to its plain meaning, as we have explained.
In summary: We learned that even though a corpse is the ultimate source of tumah, there are special stringencies regarding "a zav and his companions" - the zav, the zavah, the nidah, the woman after childbirth and the metzora: the tumah of midras, and the tumah of even mesama, in which their weight is taken into account even when it is nullified beneath the heavy stone. The stringency of even mesama continues rabbinically even after their death until the flesh decomposes, out of concern that they be confused with someone who fainted, while a non-Jew who died is pure with regard to conveying tumah by carrying. We also examined the decree of Beis Shammai, "all women die as nidos," which was decreed so that a living nidah would not be embarrassed, and the opposing view that holds "only a woman who died as a nidah is a nidah."