This is a very common nematode infection which occurs throughout the world. It is also known as pinworm or enterobiasis, as it is caused by infection with Enterobius vermicularis. The male threadworm is about 4 mm long and is rarely seen. The female is bigger at 1 cm long and a little under 1 mm in diameter. It is white and pointed at each end. It may be seen at night emerging from the anus to lay eggs. It does not appear to have any natural host other than humans and so family pets are not to blame. In the image below shows two female pinworms next to a ruler. The markings on the ruler are 1 mm apart.