An organism is generally described as parasitic when it needs a host to survive. Many organisms live a parasitic existence, including all viruses, as well as some bacteria. However, the field of "Parasitology" does not include viruses or bacteria (which are prokaryotic). It encompasses mainly eukaryotic organisms which require a host to survive, at least for part of their life cycle. Note that although fungi are also eukaryotic and can be parasitic, they come under the field of "Mycology".
Human parasites can be divided into the following major groups: Protista (protozoa), Helminthes (which are multicellular worms), and Arthropoda (insects and other related organisms). Although we generally associate parasitic diseases with the tropical areas of the world, they can, in fact, be found in most geographic regions. Parasites can also inhabit many areas in and on the human body. See the diagram, which gives examples of a few parasites and the many places they can live in the body.
This course will exclusively cover the helminth parasites that live in and around the human gastrointestinal tract. The majority, though not all, will release their eggs into the intestinal tract and can be found in the feces.
