The life cycle of threadworms                  

Threadworms live about 5-6 weeks in the gut, and then die. However, before they die the female worms lay tiny eggs around the anus (back passage). This tends to occur at night when you are warm and still in bed. The eggs are too small to see without a microscope, but cause itch around the anus.

You then scratch around the anus to relieve the itch. You often do this without realizing when you are asleep. When you scratch, eggs get onto fingers and under nails. You may then swallow some eggs if you put a finger into your mouth.

Also, threadworm eggs can survive for up to two weeks outside the body. They fall off the skin around the anus and can fall onto bedding, clothes, etc. They can then get 'wafted' in the air as you change clothes, bedding etc, and become part of the dust in a home. Some eggs may settle on food or toothbrushes. So, children may swallow some eggs at first by playing with other children who have eggs on their fingers, or from food, drink, toothbrushes, or dust that has been contaminated with threadworm eggs.

Any eggs that you swallow then hatch and grow into adult worms in the gut. So a 'cycle' of threadworm infection can go on and on.