Our story begins with Momma Pinworm, who has achieved a comfortable life in your lower intestine. Think of it in terms of the American Dream -- cute little suburban house, white picket fence, all that jazz -- except in this instance, the dream is poo. Entirely poo. Anyway, one day Momma Pinworm decides it's damn well time to live up to her name. So she waits until you're fast asleep, and she wriggles her way out of the only front door she knows: your backdoor. There, where the cooler temperatures are perfect to ensure her eggs' survival, she plops said eggs all over the puckered landscape of your shitbox before retreating to her warm, smelly lair. She'll repeat this process night after night until her life's goal is complete -- namely, squirting out as many as 16,000 young 'uns.
For them, the "circle of life" is referring to the shape of your sphincter.
And now, for the piece de resistance of the pinworm life cycle! See, pinworms are the most prolific worm infection in the USA, and in order to uphold an impressive record like that, all those eggs must get from your butthole into some other poor sod's digestive tract. How do they manage to pull off this Loki-level trickery? Well, we'll simply point out that the combination of the jelly sack surrounding the eggs and Momma Pinworm's incessant writhing plagues the sufferer with an unbearable case of butt itch. We're sure your imagination can fill in the rest, but just in case, here's a handy illustration (it's got arrows and everything!):
w4mmww a2m, must not have a propensity for shrieking in horrified disgust.