A roundworm is a type of worm with a long, tube-shaped body that's round in cross-section, like a tiny piece of spaghetti. These simple animals live almost everywhere: in soil, in water, and sometimes as parasites inside plants, animals, or even people.

Roundworms are among the most abundant animals on Earth. Scientists estimate that four out of every five animals alive are roundworms, though most are microscopic and live in dirt or ocean sediments. Some roundworms are helpful, breaking down dead material in soil and keeping ecosystems healthy. Others cause problems: certain roundworms can infect crops, pets, or livestock, and some species can make people sick if they accidentally swallow roundworm eggs from contaminated soil or unwashed food.

The most familiar roundworms to many kids are the ones that sometimes infect dogs or cats. Veterinarians prescribe medicine to eliminate these parasites, which is why pet owners often give their animals regular deworming treatments. Good hygiene, like washing your hands before eating and washing vegetables thoroughly, helps prevent roundworm infections in humans. Despite their reputation as pests, most roundworm species are harmless and play important roles in nature's recycling systems.