Above them [is] one who cohabits with a niddah, in that he transmits tumah to a bed that is beneath [him] like that which is above. Above them [is] the discharge of a zav, his spittle, his semen, his urine, and the [menstrual] blood of a niddah, in that they transmit tumah via contact and by being carried. Above them [is] a merkav, in that it transmits tumah [from] beneath a placed rock. Above a merkav [is] a mishkav, for its contact is equal to its being carried. Above a mishkav [is] a zav, in that a zav creates a mishkav, whereas a mishkav does not create a mishkav.