When you wake up in the night to scratch your bottom, it might be for all the wrong reasons !
Enterobius vermicularis is a humanopathogenic parasitic roundworm (Nematode) that currently infects about 400 million humans on this planet and is also called pinworm. Infection occurs by swallowing the eggs after either scratching one's (or somebody elses) bottom or by acquiring the eggs by some other rather unhygienic method. After ingestion, the larvae tend to hatch in the jejunum and after 1 month start migrating to the colon where they will spend the rest of their biological life and mate.
At night the gravid female will venture out of the host's anus and will lay her eggs into the perianal folds, where they will be picked up for re-infection.
Diagnosis is fun: Before opening one's bowels in the morning, a scotch tape is applied down there and after ripping it off handed to one's physician, who will put the evidence under a microscope and hunt for the eggs.
This is by far the most common helminthic infection in the western world and is pretty easily treated with pyrantel pamoate but spreads like wildfire, especially in families and kindergartens.
Wash your hands, kids!
Source:
http://www.dpd.cdc.gov/dpdx/HTML/Enterobiasis.htm
http://www.path.cam.ac.uk/~schisto/Nematodes/Enterobius.html