Ohalot, Chapter One, Mishnah Four
1) [Both] persons and vessels can be defiled through a corpse.
2) A greater stringency applies to persons than to vessels and to vessels than to persons. For with vessels [there can be] three [series of defilements], whereas with persons [there can be only] two.
3) A greater stringency applies to persons, for whenever they are in the middle of a [series] there can be four [in the series], whereas when they are not in the middle of a [series] there can be [only] three.
Section one: While both people and vessels are defiled through a corpse, there are stringencies that apply to one that don’t apply to others.
Section two: There can be a series of three defilements involving only vessels, as we saw in mishnah two. Vessel one touches the corpse, vessel two touches vessel one, and vessel three touches vessel two (but it is impure only until the evening).
However, when it comes to people, the second person has only “first degree impurity” and the person who touches him is pure, as we learned in mishnah one.
Section three: This refers to the situation in mishnah three. A vessel touches a corpse, a person touches the vessel and the vessel touches that person and all three are impure for seven days. The fourth, whether person or vessel, is also impure. But if the second element is a vessel, then the third element is impure only until the evening and the fourth element is pure, as is the case in mishnah two.