משנה על שבת 7:1

The fathers of impurity are a: sheretz, semen, [an Israelite] who has contracted corpse impurity, a metzora during the days of his counting, and the waters of purification whose quantity is less than the minimum needed for sprinkling. Behold, these convey impurity to people and vessels by contact and to earthenware by presence within their airspace, But they do not convey impurity by being carried.