A parasite is a living organism, which takes its nourishment and other needs from a host; the host is an organism which supports the parasite. The parasites included in medical parasitology are protozoa, helminthes, and some arthropods. The hosts vary depending on whether they harbor the various stages in parasitic development
DIFFERENT KINDS OF PARASITES
Ectoparasite – a parasitic organism that lives on the outer surface of its host, e.g.lice, ticks, mites etc.
Endoparasites– parasites that live inside the body of their host, e.g. Entamoebahistolytica.
Obligate Parasite – This parasite is completely dependent on the host during a segment or all of its life cycle, e.g. Plasmodium spp.
Facultative parasite – an organism that exhibits both parasitic and non-parasitic modes of living and hence does not absolutely depend on the parasitic way of life, but is capable of adapting to it if placed on a host. E.g. Naegleriafowleri
Accidental parasite – when a parasite attacks an unnatural host and survives. E.g. Hymenolepisdiminuta(rat tapeworm).
Erratic parasite – is one that wanders in to an organ in which it is not usually found.E.g. Entamoeba histolyticain the liver or lung of humans.
Most of the parasites which live in/on the body of the host do not cause disease (Non-pathogenic parasites).
In Medical parasitology we focus on most of the disease causing (pathogenic) parasites.
DIFFERENT KINDS OF HOSTS
Definitive host – a host that harbors a parasite in the adult stage or where the parasiteundergoes a sexual method of reproduction.
Intermediate host – harbors the larval stages of the parasite or an asexual cycle ofdevelopment takes place. In some cases, larval development is completed in twodifferent intermediate hosts, referred to as first and second intermediate hosts
Paratenic host – a host that serves as a temporary refuge and vehicle for reaching anobligatory host, usually the definitive host, i.e. it is not necessary for the completion ofthe parasites life cycle.
Reservoir host – a host that makes the parasite available for the transmission toanother host and is usually not affected by the infection.
Natural host – a host that is naturally infected with certain species of parasite.
Accidental host – a host that is under normal circumstances not infected with theparasite.
There is a dynamic equilibrium which exists in the interaction of organisms. Any organism that spends a portion or all of its life cycle intimately associated with another organism of a different species is considered as Symbiont (symbiote) and this relationship is called symbiosis (symbiotic relationships).