Far above the blue marble we've come to call our home, a strange species of parasitic alien creatures was approaching. As they floated into our atmosphere they descended upon humanity. It wouldn't be long until our world fell prey to these mysterious invaders. They came in great hordes of several dozens and attacked without warning at lightning speed. They attached themselves around our waists and between our legs, dissolving the clothing they'd engulfed and replacing it with themselves. It was then we realized what these alien invaders truly were.
The strange creatures resembled the cottony disposable diapers worn by infants, although these were sized for adults to wear. Once latched on, the creatures puffed out and inflated to the size of large yoga balls, about two-and-a-quarter feet wide. Their grip was too strong for the victim forced to wear it to remove alone, and their inflated girth forced their stances wide and made walking normally much more difficult. The creatures had small fangs that bit into their hosts once attached, and injected their victim with a potent venom that had three primary effects. First, it affected your bowels and your bladder, dissolving what continence you'd have developed up to that point, forcing you to excrete your bodily waste involuntarily. This is because the creature actually derived nutrients from human waste and digested it through absorption. Second, the venom would slowly work its way up your nervous system, reaching your brain and forcing subtle, yet compounding mental changes upon you. These changes would affect your speech patterns, giving you an infantile lisp. It would also affect your personality and mannerisms, making them more infantile as well, and making them increasingly more reluctant to give up their diaper. Last, the venom would affect your metabolism, forcing your body to retain more calories and start developing more "baby fat" as people called it.
The diaper parasites would effectively leech off of your bodily excretions forever until you passed away of old age, whereby then it will start laying eggs before shriveling away and dying itself. These eggs would hatch in a matter of days and start seeking out new hosts to latch onto and feed off of until it was time for them to lay their eggs and die. These creatures could be removed with outside help, and doing so would actually kill the parasite, but chances were the host's mind would be under complete control of the alien by then and they would put up resistance to having their diaper taken off. Once the alien is removed the victim's mind will swiftly return to normal, but they will have to re-train themselves to hold their continence like they used to.