A nematode is an unsegmented worm belonging to the phylum Nematoda. They are the most numerous multicellular animals on earth. The body of a nematode is elongated and cylindrical. Most nematodes are tiny and live in huge numbers in soil, water, plants and animals. Nematodes such as roundworm, pinworm and hookworm are parasites that live in the blood and lymphatic system and cause different types of disease.

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Nematodes often live in soil.

Intestinal Roundworm

Intestinal roundworm also is called ascariasis. Infection with ascariasis is the most common type of human worm infection, according to Centers for Disease Control and Prevention. Intestinal roundworm can live in the small intestine. Adult female roundworms can reach a length of 12 inches. Infection is most common in tropical areas with poor sanitation.

Pinworm

Pinworm infection is caused by a small white nematode. Infections occur worldwide. Symptoms are usually mild and may be nonexistent, but may include intense itching around the anus. Pinworm eggs can survive on clothing or bedding for up to three weeks, according to Centers for Disease Control and Prevention.

Trichinosis

Trichinosis is caused by eating raw or undercooked meat of an animal infected by the larvae of the nematode Trichinella. Initial symptoms are diarrhea, nausea, fever, fatigue and abdominal discomfort. Secondary symptoms include aching joints, muscle pain, headache, cough, fever and chills. Abdominal symptoms appear one or two days after infection. Other symptoms start a few weeks after consuming contaminated meat. Symptoms can be mild or very severe.

Hookworm

Hookworms cause mild abdominal pain and diarrhea. Infection with hookworm usually occurs in tropical and subtropical climates. Hookworms live in soil, so infection can be contracted by contact with contaminated soil. This can happen by walking barefoot or by accidentally swallowing contaminated dirt. The first sign of infection is usually itching or a rash at the infection site. Mild infection may cause no symptoms, but a more serious infection can cause loss of appetite, weight loss and diarrhea.

Lymphatic Filariasis

Parasitic worms can invade the lymphatic system. Lymphatic filariasis, also called elephantiasis, is a disease caused by three species of thread-like nematode worms. Male and female worms work together to form nests in the lymphatic system. The lymphatic system is an important part of the body's immune system. This type of infection can cause severe and permanent disability. It is spread from one person to another by mosquitoes. Malfunction of the lympthatic system may result in fluid collection and swelling of the legs, arms or genitalia.