More 15,000 species of nematodes are parasitic. Nematoda, from Ancient Greek, means "thread-like ones." I suppose a small, thin worm is like a thread. Unfortunately, the human parasitic species -- like ascarsis shown in the photo -- is not as innocuous as a thread. Intestinal nematode infections account for almost half of the malnutrition in the world. (Photo courtesy of the Orange County Public Health Laboratory, Santa Ana, CA.)
Adult female worms can grow to be almost a foot in length. Imagine a bundle of these things in your intestines eating the nourishment you supply. They are going to grow, and you won't. It's rare that you would ever get this disease in the USA. But children in tropical an subtropical areas are at great risk if they drink unclean water, have inadequate sanitation, or poor hygiene.
Over a billion people are walking around with these worms. You can make a difference by donating to Blue Planet Run Foundation. Clean water is needed for good hygience.