Ohalos, Chapter 1, Mishnah 1. B'ezras Hashem Yisborach, we begin a new mesechta. Its opening mishnah presents a kind of table of contents: how far does the tumah of a meis travel? Sometimes only two are made tomei, sometimes three, and sometimes even four. The mishnah lists the counts first, and then begins to explain the smallest case. The mishnayos that follow will fill in the rest.
Two who are tomei from a meis
The mishnah opens with "Shnayim tmei'im b'meis." There are circumstances in which the chain of tumah runs only two deep. Someone touches a meis and is thereby tomei, and he is able to pass tumah along to one further person, or to one further keili, and at that point the transfer halts. Whoever or whatever received the tumah at that second stage has nothing left to hand on to anyone else.
Picture it with names. Reuven puts his hand on the meis, and Reuven is tomei. Shimon then comes into contact with Reuven, and Shimon is tomei. The same would be true of a keili: if a vessel comes into contact with Reuven, that keili is tomei. But there is no step three. If Levi afterwards comes into contact with Shimon, nothing happens to Levi at all.
Two who are tomei, but not equally
Although Reuven and Shimon are both tomei, their statuses are far from identical, and the mishnah makes the difference explicit: "Echad tomei tumas shiv'ah," one of the two bears a tumah lasting seven days, namely Reuven, whose own hand met the meis, "v'echad tomei tumas erev," while the second man's tumah expires with the coming of night.
For Reuven those seven days involve more than simply waiting. On the third day, and again on the seventh, he must be sprinkled with the mei chatas, the water into which parah adumah ashes were placed. Shimon's position is considerably lighter: he immerses, waits for the sun to set, and he is tohor.
Three, and even four
Two links are not the only possibility. The mishnah continues: "Shlosha tmei'im b'meis." Sometimes the tumah reaches as far as a third link in the chain, so that beyond Reuven and Shimon there is one more. (I deliberately say "one more" rather than "one more person," because the coming mishnayos will distinguish between an adam and a keili, and that distinction will prove significant.) For such a case the mishnah supplies the breakdown: "shnayim tmei'in tumas shiv'ah," the earlier two bear a tumah of seven days, "v'echad tomei tumas erev," while the last of the three is tomei only until the evening.
Nor does the count stop at three. "Arba'ah tmei'im b'meis." There are situations in which a fourth is drawn in as well: whatever touched the meis, whatever touched that, whatever touched the next in line, and then one further removed still. Of these the mishnah states "shlosha tmei'in tumas shiv'ah," the first three all bear the seven day tumah, with the sprinkling of the mei chatas on days three and seven, "v'echad tomei tumas erev," while the last of them is tomei only until nightfall.
Do not let the list unsettle you. At this stage the mishnah is only giving us the broad outline. Every one of these cases will be spelled out in the mishnayos that follow.
Keitzad shnayim: the case of two explained
Having listed the counts, the mishnah returns to the smallest of them and asks "keitzad shnayim," in what circumstance is there a first and a second and nothing beyond? It answers: "Adam hanogei'a b'meis." A meis holds the highest rank in all of tumah, an avi avos hatumah. Reuven, by touching it, thereby holds the rank of av hatumah, and so "v'tomei tumas shiv'ah," seven days of tumah. "V'adam hanogei'a bo": Shimon, whose contact is with Reuven, falls one rank and becomes a rishon l'tumah, and hence "v'tomei tumas erev," meaning that his tumah ends when the day ends, and at nightfall he is tohor.
What, then, have we learned? A scenario of a first and a second, in which a third remains altogether unaffected. The one with direct contact, Reuven, holds the rank of av hatumah. The one who drew tumah from him, Shimon, holds the rank of rishon l'tumah. A third party takes nothing whatsoever. There are other scenarios in which a third does contract tumah, and even ones in which a fourth does. Identifying where those scenarios are found is precisely the work of the mishnayos ahead, b'ezras Hashem Yisborach.
The pesukim behind the levels
You may, if you like, proceed directly to the next mishnah. Still, it repays the effort to see where these two levels come from. What tells us that one who touched a meis bears a tumah of seven days? A possuk: "Hanogei'a b'meis l'chol nefesh adam," and of such a person the Torah declares "v'tomei shiv'as yamim." The corpse itself stands at the top of the ladder as an avi avos hatumah; whoever touches it, whether an adam or a keili, takes on the rank of av hatumah, and that rank carries with it a tumah of a full seven days.
And what tells us that the next one in line holds only the rank of rishon l'tumah, tomei merely for the day? The possuk "v'hanefesh hanoga'as," which speaks of one who touched the man who had handled the corpse: in our illustration, Shimon, whose contact was with Reuven, while Reuven's contact was with the meis. Regarding him the Torah says "titma ad ha'arev," his tumah lasts until the evening. He immerses, the sun sets, and he is tohor. Shimon accordingly stands as a rishon l'tumah, and from that rank he has no power to convey tumah onward to a Levi or to any keilim.
An important qualification from the Bartenura
Azoy zogt der Bartenura: this entire picture presumes that Reuven had already released the meis before Shimon touched him. Should Shimon reach him while his hand still rests on the corpse, Shimon too takes on the rank of av hatumah, a matter that will be developed later on. But if Reuven had let go beforehand, and the touching happened only afterward, Shimon drops to the rank of rishon l'tumah. No seven day period applies to him at all, only that single day: he immerses in the mikvah, and then he is tohor.