Toxoplasma gondii can infect humans too, and perhaps even jigger our brains, as Anne whimsically suggests in the title of her e-mail: Thoughts are worms.
Speaking of worms, consider the nematodes, a spectacularly diverse and abundant family of tiny critters that live just about everywhere. I always liked this description by N. A. Cobb that appeared in the 1914 edition of the Yearbook of the United States Department of Agriculture:
If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes and oceans represented by a thin film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.Imagine, Anne, that every atom of your body suddenly vanished except for your bugs. For an instant, until it dispersed, there would be left behind a ghostly replica of yourself, made entirely of living creatures, from bacteria to mites. Eyebrows. Scalp. Armpits and groin. Toenails and teeth. The caves of your nostrils, mouth and ear canals. The curling Amazon of your intestines. Billions upon billions of now homeless organisms exquisitely remembering your form.