Pinworm infection is very common. They are small, thin worms (about 4 cm long). Pinworms are especially common where children sleep or play together in crowded conditions. Adult female worms pass out the anus at night to lay eggs on the perineum. Eggs are swallowed from contaminated fingers, clothing or bed linen. Pinworms are common even where hygiene and sanitation are of a high standard. They are also known as threadworms.