The avos of tumah [are]: A sheretz, a seminal discharge, one who contracted tumah from a corpse, a metzora during the days of his counting, and purification waters that are insufficient to sprinkle. These render people and utensils tamei via contact, and an earthenware utensil via airspace, but they do not render [one] tamei via being carried.

Above them [are]: A neveilah, and purification waters that are sufficient for sprinkling, in that they render a person tamei via carrying to render clothing tamei via contact, but clothing is spared [from tumah] when there is [only] contact.

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.

Above the zav [is] a zavah, in that she transmits tumah to any man who cohabits with her. Above the zavah [is] a metzora, in that he transmits tumah via entry [into a covered area]. Above the metzora [is] a bone the size of a barley grain, in that it transmits a tumah of seven [days]. More stringent than all of them [is] a corpse, in that it transmits tumah via an ohel, a means through which all the others do not transmit tumah.