Well, I'm not exactly sure what playground sand is made from, but I'm guessing it's not very digestable and may have had some parasites in them. There are a lot more ways for you to catch parasites anyways. You could eat meat, unwashed fruits, veggies, leafs, not wash your hands before eating, or picking up eggs from your pet.
I would look at certain problems you had in your childhood that could link to having parasites. Where you often constipated? If you are not passing digested material through the intestines quick enough, the parasitic eggs can hatch and become comfortable either in your blood or your intestinal walls.
This could give you an idea of how you picked up parasites, although the ways are endless. That is why after completing a parasite cleanse, it should be maintained every 6 months or so.