Once apon a time in a far off land ( your bowel) there lived a couple of threadworms called Nigel and gertie.
Now Nigel hadn't known gertie very long but unfortunately circumstances dictated that they had to cohabit, but it was ok by them as its a short term thing in 13 weeks both will be dead. And anyway it was not there fault the eggs that there feckless mother left behind ended up on the same door handle
Now both of them are night owls and like to go exploring around there hosts bum, and gertie lays lots of eggs. She's rather clever and surrounds these eggs with a irritant mucus.
The host in her sleep scratches her bum and the eggs get under her nails. The next day she is a bit late to work I'm guessing she overslept so didn't wash propaly, she jumps on a bus to Baker Street ( that's where she works) as she does so she deposits an egg on the hand rail.
Egg ends up on another passengers hands he then picks his nose and eats it egg finds its way to the intestine because they are invisible to the naked eye he obviously didnt see it apon bogey inspection.
Egg hatches very quickly and takes qabout 7 weeks to reach adulthood
( in the interests of impartiality some bloke called cook thinks its only 4 weeks)
After that time gerties and Nigel's baby lets call her pam, moves very fast 12 cmh and sets about loading her whole body with more eggs pam will have about 15000
About 5-7 weeks after the egg that became known as pam will start laying these eggs.some will get huffed and puffed out ( with out epidural) but any left by about 13 weeks get laid by pam dying and disintegrating. Sadly pans boyfriend who weirdly is also her son only lives for 7 weeks. Pam manages 13 poor soul.
There babies then go on and do there stuff. They are much harder to evict due to the eggs being able to live in a warm moist environment for up to 3 weeks.