Infestation with Enterobius vermicularis, the threadworm or pinworm. It is the most common of all the intestinal parasites in Britain, affecting children particularly. About 6–12 mm (¼–½ inch) in length and they resemble little pieces of thread. They live in considerable numbers in the lower bowel. They are generally harmless, not causing symptoms except for irritation around the anus or vulva, especially at night when the female worm emerges from the anus to lay its eggs and then die. The most effective form of treatment is either viprynium embonate or piperazine citrate, which is usually prescribed for the whole family. Bedclothes must then be laundered.