Mischna zu Wajikra 14:46

If a person who was clean put his head and the greater part of his body inside an unclean house, he becomes unclean. And if an unclean man put his head and the greater part of his body inside a clean house he causes it to be unclean. If he put three fingerbreadths square of a clean cloak into an unclean house, the cloak becomes unclean; And if he put even the size of an olive of an unclean [cloak] into a clean house, the house becomes unclean.