They are present in house dust and stick to bedding, clothing etc. They tend to lie in the itestine and around the anus. Suggest an appointment with your GP
Not really sure where they originate but any chemist can give you an over the counter remedy which clear them in no time. However it is really important to exercise good hand hygiene as they are usually passed from hand to mouth. They are really common in children who don't wash their hands well after using the bathroom, with intense itching around the bottom particularly at night, Not pleasant!
Threadworms live in the gut (intestine) for about 2 months and then die...BUT before they die, the female lays eggs which are deposited around the anus which causes itching and subsequent scratching by the host. The eggs fall of and lie in the bedclothes as described by the posters above.
Drugs will kill the threadworm, but not the eggs. so scrupulous hygiene is necessary in the treatment.