Threadworms are small worms that live in the lower bowel (gut) . The reproduce by crawling out to the anus (back passage), and laying eggs around the outside of the anus. They usually do this a night.
Threadworms are usually associated with an itchy anus (itchy bottom). The mucous that the threadworms use to stick the eggs to the skin is the main cause of the itchy feeling. This itchy feeling is an important way the Threadworm promotes it's spread. When people (particularly kids) scratch the itch they pick up eggs on their fingers, and under their finger nails. The eggs are then carried around and transfered on to things they touch even though they cannot be seen.
Because Threadworm eggs can survive for around 2 weeks before hatching, and because the eggs are sticky they can be passed between people and things quiet easily.
The best way to prevent the spread of Threadworms is careful hand washing particularly if you've just had a scratch or been to the toilet. You should always wash your hands before eating, no matter what.
Treating Threadworms is really easy. Usually everyone in the house takes a single tablet (they even taste like chocolate to get them past kids) at bedtime. The dose should be repeated after 2 weeks to break the re-infestation cycle. Washing all the bed sheets, and used underwear in a hot wash the morning after treating is also a good way to reduce re-infestation risk.
Although Threadworms aren't usually the cause of serious ill health, as they encourage scratching of the anal area there is risk of oral-faecal contamination, in otherwords transfer of bacteria that cause illness from the back passage to the mouth if hand hygine is poor.