הלכות מטמאי משכב ומושב

We have already explained that a zav and the others in that category impart impurity to an article on which one lies, sits, or rides.
An article on which one lies and one on which he sits are essentially the same. Why then did the Torah mention them separately? To teach that although a seat is fit for sitting and a couch is fit for lying, whether one sits on a couch or lies on a seat, he causes it to become impure. Therefore I will mention a surface on which one lies alone and it should be understood that the same applies to a surface on which one sits.
What is meant by a surface on which one rides? The surface on which one sits when riding, e.g., a donkey's packsaddle or a horse's saddlecloth.

Any article on which one lies or rides to which a zav or the like imparted impurity becomes a primary source of impurity according to Scriptural Law. It imparts impurity to a person and keilim when touching them and to a person when being carried by them.
What is the difference between the laws pertaining to an article on which one lies and one on which one rides? With regard to touching an article on which one lies, if a person touches clothes and other keilim while he is still touching the impure article, he imparts impurity to them and makes them a primary derivative of impurity. One who touches an article on which one rides, by contrast, does not impart impurity to clothes and keilim even when he touches them while touching the impure article. Nevertheless, one who carries either an article on which one lies or rides imparts impurity to clothes and other keilim until he severs contact with the source of his impurity.
What is the source that teaches that a distinction is made between touching and carrying with regard to the impurity of an object on which one rides and no such distinction is made with regard to the impurity of an object on which one lies? With regard to an object on which one lies, Leviticus 15:5 states: "A man who touches his couch.... shall launder his clothes," while with regard to an object on which one rides, ibid.: 9-10 states: "Any object on which he will ride... anyone who touches anything that will be beneath him will be impure until the evening." In the latter instance, it is not said that he must launder his garments. This teaches that he does not impart impurity to his garments when touching the source of impurity. And it is written ibid.: "One who carries them shall launder his garments." This applies equally to one who carries an article on which one rides and an article on which one lies.
Thus it can be concluded that a zav, a zavah, a nidah, and a woman after childbirth, the articles on which they lie, their saliva, their urine, the blood of a nidah, a zavah, and a woman after childbirth, and the discharge of a zav are all primary sources of impurity. If a person touches any of these or carries them, he imparts impurity to his clothes and other keilim while he is touching them or carrying them. He does not, however, impart impurity to another person or to an earthenware container. For any entity that imparts impurity to a human imparts impurity to an earthenware container and any entity that does not impart impurity to a human does not impart impurity to an earthenware container.
Thus it can be concluded that whenever it is said about an impure person: "He shall launder his clothes," he imparts impurity to clothes that he touches as long as he does not separate from the entity imparting impurity to him. He makes those garments primary derivatives of impurity, like he is. He imparts impurity to other keilim like garments with the exception of earthenware containers. For he does not impart impurity to another person or to earthenware containers even though he has not separated from the entity imparting impurity to him.
Whenever, by contrast, Scripture does not say about an impure person: "He shall launder his clothes," his status is the same before he separates from the entity imparting impurity to him as it is afterwards. He does not impart impurity to garments, because he is merely a derivative of impurity. Needless to say, he does not impart impurity to another person or to earthenware containers. Therefore a person who touches an article on which a zav rode does not impart impurity to garments when touching it. One who carries such an article, by contrast, imparts impurity when carrying it, as we explained.

All keilim that rest above a zav or the like are called madaf. They are considered as articles that he touches and they do not impart impurity to another person or to an earthenware container. They do, however, impart impurity to food or liquids like other derivatives of impurity. The impurity of madaf is a Rabbinic decree.

When a zav, a zavah, a nidah, a woman after childbirth, or a person afflicted by tzara'at die, they impart impurity to an object on which they lied or rode after their death just as they would during their lifetime until their flesh decomposes. This is a Rabbinic safeguard, decreed lest one of these individuals lose consciousness and it appear that they died, but they did not actually die. Thus a surface on which one of these lie or ride after death is a primary source of impurity by Rabbinic decree. When, by contrast, a gentile dies, a surface on which he lies after death is not considered as a source of impurity, for even when he is alive, he imparts impurity only due to Rabbinic decree, as we explained.

Whenever you hear the terms "the impurity of an object upon which one lies or rides," the intent is not necessarily that the impure person touch the object on which one lies or rides while resting on them. Instead, even if there are large stones above the entity made to lie or ride on and the zav rested on the stones from above, the surface on which one lies or rides becomes a primary source of impurity. Even when there are 1000 surfaces on which a person lies or rides one on top of another and a stone is on the uppermost and a person who imparts impurity to an object on which he or she lies or sits on top of the stone, everything becomes impure. Both the article on which one lies that touches the earth and the uppermost one become primary sources of impurity when they are affected by the zav's lying down.
Similarly, if one of the individuals who impart impurity to an object on which he or she lies or sits was below, there was a stone on top of him, and food, liquids, keilim, and/or a person on the stone, one on top of the other, they are all impure and are considered as primary derivatives of impurity. Whether a k'li, food, a liquid, or a person touches a zav or the k'li, food, liquid, or person was on a stone that was above the zav, there is no difference. In both instances everything is considered as a derivative of impurity and does not impart impurity to a person or to keilim. The only exception is a person who is above a zav who is still in contact with the zav who imparted impurity to him. He imparts impurity to other keilim, as we explained.

When food, liquids, and keilim that are not made to lie, sit, or ride upon were below other articles and a zav or the like sat above them without touching them, they are all pure. If, however, a person was below and a zav was above, he is impure even if he does not touch him, because he is carrying the zav, like anyone who carries an article that imparts impurity when carried. The one carrying it becomes a primary derivative of impurity.
Thus one can conclude that anything that is above a zav is impure, whether a person, articles on which one lies, sits, or rides, all other keilim, food, and liquids. Everything is a primary derivative of impurity. Everything that is below a zav and does not touch him is pure except a person or an article made to lie, sit, or ride upon. There is a difference between them. The person is a derivative of impurity and the article made to lie, sit, or ride upon is a primary sources of impurity, as we explained.