Roundworms are long, round parasites that may infect humans and cause health problems. Most often, people ingest roundworm larvae or eggs after touching soil, although they may enter directly through the skin, according to the National Institute of Allergy and Infectious Diseases.

Roundworms include more than 15,000 known species and up to half a million unknown species. Most do not cause disease, but several species can infect humans, causing symptoms that range from mild to severe. Although serious roundworm infections are uncommon in the U.S.