Bava Metzia 105B

עלו באילן שכוחו רע, ובסוכה שכוחה רע ⁠– טמא.

If a zav and a ritually pure person climbed a tree that has little strength, which shook as they climbed it, or if they climbed onto a branch that has little strength, the ritually pure person is rendered ritually impure. One of the ways a zav imparts impurity is by movement, and here the zav is viewed as having moved the pure person.

היכי דמי אילן שכוחו רע? אמרי דבי רבי ינאי: כל שאין בעיקרו לחוק רובע. היכי דמי סוכה שכוחה רע? אמר ריש לקיש: כל שנחבאת בחזיונה.

The Talmud asks: What are the circumstances of this tree that has little strength, i.e., how is a tree with little strength defined? The rabbis of the school of R' Yannai say: It is any tree whose trunk is not broad enough that one can hollow out a vessel of 1/4th-kav from it. What are the circumstances of a branch that has little strength? Reish Lakish said: It is any branch concerning which its circumference can be hidden, i.e., inserted, in a person’s fist. A branch of this size is generally not strong enough to hold two people without shaking.

תנן התם: המהלך בבית הפרס, על גבי אבנים שיכול להסיט, על האדם ועל הבהמה שכוחן רע ⁠– טמא.

We learned in a Mishnah elsewhere (Oholot 18:6): With regard to one who walks in an area in which uncertainty exists concerning the location of a grave or corpse [beit haperas], if he treads over stones that he can move as he walks, raising concerns that he might have moved a bone of a corpse and thereby rendered himself impure, or if he was in that location, on the back of a person or riding on an animal that had little strength, he is impure, as he is considered to have moved the impurity himself.