The females come out at night to lay eggs in the rectal area. If the eggs are left to hatch, the immature larvae will crawl back through the anus and wind up back in the colon. Occasionally, in girls, the larvae will crawl into the vagina instead. The paddle is pressed against the anal skin of the patient. If pinworm eggs are present, they will show up on the paddle.
Most sources say pinworms are usually harmless.
Some even say that it doesn't need to be treated, unless the symptoms really bother the patient. However, this is not something I'd want to have. Fortunately, the treatment is very simple, consisting of two doses of medicine two weeks apart. The majority of sources recommend treating all family members at once, even if only one person shows signs of infestation. Most doctors will also recommend washing all the family linen and night clothes in hot water, to remove any eggs that might be floating around.