In some cases, various stages have developed the ability to resist adverse environmental conditions. This sometimes involves only partially shedding the cuticle at a molt, or the ability to reduce the rate of metabolic functions or alter metabolic pathways for a period of time. In parasites of vertebrates, life cycles are typically termed direct if an intermediate host is not required or indirect if an intermediate host is a part of the life cycle. It is not uncommon for a single host to have several kinds of parasitic nematodes in various locations throughout the body. Many vertebrate parasites follow specific migratory pathways in the host from infection to final destination.
Some plant parasites inhabit specific areas of roots while others are more general browsers.
Humans frequently have a desire to categorize things into discrete groupings based on a set of characteristics. Approximately 15 terms have been proposed to categorize the types of associations that exist between two separate species and we will utilize some of them to simplify our discussion of nematodes. Animal associations, however, do not exist in such discrete categories but rather as a spectrum or a continum from free-living to parasitic with gradations in between. Commensalism is the loosest and least obligatory of the interspecific associations as both partners are thought to benefit and are believed to be at a biological advantage over those individuals of the same species not in association. Mutualism is distinguished from commensalism in that the association is more intimate and essential to the associates.