When a house extends over an afflicted house or a tree extends over an afflicted house, someone who enters the outer one is tahor, these are the words of R’ Elazar ben Azariah. Said R’ Elazar: If one of its stones renders tamei upon entering, the house itself should certainly render tamei upon entering.

[If] someone tamei is standing under a tree and someone tahor passes [underneath the tree, the tahor person] becomes tamei. [If] someone tahor is standing under a tree and someone tamei passes [underneath, the tahor person] remains tahor. If [the tamei person] stops, [the tahor person becomes] tamei.