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Alternatively, the immature (larval) states can cause disease through their infection of various body tissues.

Roundworms (nematodes) – the adult forms of these worms can reside in the gastrointestinal tract, blood, lymphatic system or subcutaneous tissues.

The acanthocephala are thought to be intermediate between the cestodes and nematodes.

Thorny-headed worms (acanthocephalins) – the adult forms of these worms reside in the gastrointestinal tract.

Flatworms (platyhelminths) – these include the trematodes (flukes) and cestodes (tapeworms).

There are three main groups of helminths (derived from the Greek word for worms) that are human parasites: In their adult form, helminths cannot multiply in humans. Like protozoa, helminths can be either free-living or parasitic in nature.

Helminths are large, multicellular organisms that are generally visible to the naked eye in their adult stages. Sporozoa – organisms whose adult stage is not motile e.g., Plasmodium, CryptosporidiumĪn adult Ascaris lumbriocoides worm.Ciliophora – the ciliates, e.g., Balantidium.Mastigophora – the flagellates, e.g., Giardia, Leishmania.The protozoa that are infectious to humans can be classified into four groups based on their mode of movement:

Protozoa that live in the blood or tissue of humans are transmitted to other humans by an arthropod vector (for example, through the bite of a mosquito or sand fly). Transmission of protozoa that live in a human’s intestine to another human typically occurs through a fecal-oral route (for example, contaminated food or water or person-to-person contact). They are able to multiply in humans, which contributes to their survival and also permits serious infections to develop from just a single organism. Protozoa are microscopic, one-celled organisms that can be free-living or parasitic in nature. A microscope is necessary to view this parasite.