Pinworm. Scotch Tape applied on the anal area first thing in the am will pick up deposited eggs, and diagnosis can be made by looking the tape with a microscope. The eggs are not laid in the stool but are stuck on the anus, readily available to be picked up on fingers to be spread.
Being human usually suffices.
A 100 mg of mebendazole repeat after 2 weeks.
Pinworm eggs have been found in fossil poo, carbon dated to 7837 BC in western Utah, but no fossil scotch tape.