Helminths, comprising both Platyhelminthes and Nemathelminthes, in contrast, are macroscopic, multicellular worms possessing differentiated tissues and complex organ systems; they vary in length from more than 1 m to less than 1 mm. The majority of both Protozoa and helminths are free living, play a significant role in the ecology of the planet, and seldom inconvenience the human race. The less common disease-producing species are typically obligate parasites, dependent on vertebrate hosts, arthropod hosts, or both for their survival. The majority of parasites are perfectly happy living in a commensalistic relationship with their host, producing little or no injury. Of importance to us are those that disturb this relationship, leading to pathogenesis and, occasionally, to death of both the host and parasite.

They are opportunists by nature and exploit environmental niches and lifestyles within their hosts that suit their individual needs. Many have high prevalence rates, given the right set of circumstances, and may cause significant morbidity and mortality. All have exceedingly complex life cycles. ¹