Parasitology is the study of parasites, their hosts, and the relationship between them. As a biological discipline, the scope of parasitology is not determined by the organism or environment in question, but by their way of life. This means it forms a synthesis of other disciplines, and draws on techniques from fields such as cell biology, bioinformatics, molecular biology, immunology, genetics and ecology.
The parasitic mode of life is the most common on the planet, with representatives from all major taxa, from the simplest unicellular organisms to complex vertebrates. Every free-living species has its own unique species of parasite, so the number of parasitic species greatly exceeds the number of free living species.
The study of these diverse organisms means that the subject is often broken up into simpler, more focused units, which use common techniques, even if they are not studying the same organisms or diseases. Much research in parasitology falls somewhere between two or more of these definitions. In general, the study of prokaryotes fall under the field of bacteriology rather than parasitology.
One of the largest fields in parasitology, medical parasitology is the study of those parasites which infect humans. These include unicellular organisms such as Plasmodium spp., the organism which causes malaria, Leishmania donovani, the organism which causes leishmaniasis and multicellular organisms such as Schistosoma spp., Wuchereria bancrofti and Necator americanus. Medical parasitology can involve drug development, epidimiological studies and study of zoonoses.
The study of parasites which cause economic losses in agriculture or aquaculture operations, or which infect companion animals. Examples of species studied are Lucilia cericata, the blowfly, which lays eggs on the skins of farm animals, the maggots hatch and burrow into the flesh, distressing the animal and causing economic loss to the farmer, Otodectes cynotis, the cat ear mite, responsible for Canker, and Gyrodactylus salaris, a monogenean parasite of salmon, which can wipe out populations which are not resistant.
The study of parasites can reveal information about their relationship with the host (eg. influences on mortality rate) and relationships between different populations of host species. This technique is commonly applied in fisheries biology, where parasites are used to identify different populations of the same fish. This forms an interesting application of the Red Queen hypothesis.
The huge diversity within the parasitic animals creates a challenge for biologists to describe and catalogue them. Recent developments in using DNA to identify separate species and to investigate the relationship between groups at various taxonomic scales has been enormously useful to parasitologists, as many parasites are highly degenerate, disguising relationships between species.