Nematodes, or roundworms, have an estimated 80,000 species that are known to be parasitic. The general morphology of these worms is consistent with their name; they are usually long and cylindrical in shape.
One of the most infamous nematodes is Trichinella spiralis, a parasite that lives its larval stage encysted in the muscle tissue of animals, including swine, and make their way into the intestinal tissue of humans who happen to digest infected, undercooked pork.
The largest parasitic roundworm, common among humans living in tropical developing countries, is Ascaris lumbricoides. This roundworm can grow up to 14 in (35 cm) in length within the small intestine of its host.
One roundworm, Enterobius vermicularis, or pinworm, actually thrives in more temperate climates. This relatively small roundworm is not limited to humans living in relative poverty, but is known to infect the well-off just as easily.
Adult worms of Wuchereria bancrofti live in the blood and lymph of the host. Elephantiasis, characterized by extreme enlargement of a host's extremities, is the rare but dramatic result of the host body's defensive reaction to the presence of the worms.
Known to dog owners Dirofilaria immitus, or heart-worm infection, if left untreated, can kill a dog as the worms infect the heart tissues and eventually weaken the cardiac muscles to the point of failure.