Mischna zu Wajikra 13:46

If one had a bright spot and it was cut off, he becomes clean; If he cut it off intentionally: Rabbi Eliezer says: [he is clean] only after another nega arises in him and he has attained cleanness after it. But the sages say: only after it has spread over all his body. If it was on the tip of one's foreskin, he should be circumcised.

If an unclean person stood under a tree and a clean person passed by, the latter becomes unclean. If a clean person stood under a tree and an unclean one passed by, the former remains clean. If the latter stood still, the former becomes unclean. Similarly in the case of a leprous stone he remains clean. But if it was set down he becomes unclean.