Leviticus 5:2-3

2Or if a person touches 1any unclean thing, whether a carcass of an unclean beast or the carcass of unclean cattle or a carcass of unclean swarming things, though it is hidden from him and he is unclean, then he will be guilty.

3Or if he touches human uncleanness, of whatever sort his uncleanness may be with which he becomes unclean, and it is hidden from him, and then he comes to know it, he will be guilty.