Zavim, Chapter Three, Mishnah One
1) A zav and a clean person who sat together in a boat, or on a raft, or rode together on a beast, even though their garments did not touch, behold they are impure through midras.
2) If they sat together on a plank, on a bench or on a bed-frame, or on a beam, when these were not fixed tightly,
a) [Or] if they had both climbed a weak tree, or [were swaying] on a weak branch of a strong tree;
b) Or if they were both [climbing] on an Egyptian ladder which was not secured by a nail;
c) Or if they sat together on a bridge, rafter or door, not secured by clay, they are unclean.
d) Rabbi Judah says that they are clean.
Section one: In this case the zav and a clean person are traveling together on a boat, raft or animal. The clothes of the pure person are defiled by the zav even if they didn’t touch. There are two possible explanations for why. First of all, since these things are all moving, it is possible and even likely that the zav did indeed lean upon the clean person’s clothing. Alternatively, the clean person could have caused the ship, raft or animal to lean in his direction which would cause the zav to go up. This is “hanging” explained in yesterday’s mishnah.
Section two: In all of these cases, the zav and the clean person sit on something loose. Since the zav’s moving would cause the clean person to move as well, the clean person is defiled. This is called defilement through shifting.
An alternative explanation is that this is the same as the case in section one. The clean person might cause the zav to go up and this is considered “hanging.”