If one house overshadowed another house that had a nega – and, similarly, if a tree overshadowed a house with a nega – then one who enters the outer one remains ritually clean; this is the opinion of Rabbi Elazar ben Azariah. Rabbi Elazar (not the same person) said that if one stone of the house conveys impurity by entering then the house itself should certainly convey impurity by entering.
If a ritually-unclean person stood under a tree and a ritually-clean person passed by, the clean person is rendered unclean. If the clean person stood under the tree and the unclean person passed by, the clean person remains clean; if the unclean person stood in one place, the clean person is rendered unclean. The same is true regarding a nega-infected stone: if one passes under a tree with it, a ritually-clean person remains clean; if the stone is put down, the clean person is rendered unclean.