Nematodes are nonsegmented cylindric worms ranging from 1 mm to 1 m in length. Nematodes have a body cavity, distinguishing them from tapeworms and flukes. Depending on the species, different stages in the life cycle are infectious to humans. Hundreds of millions of humans are infected with nematodes; the most common are Ascaris, hookworms, and Trichuris.

Last full review/revision December 2009 by Richard D. Pearson, MD