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'Trypanosomes' are a group of kinetoplastid protozoa distinguished by having only a single flagellum. All members are exclusively parasitic, found primarily in insects. A few genera have life-cycles involving a secondary host, which may be a vertebrate or a plant. These include several species that cause major diseases in humans.
The most notable trypanosomal diseases are trypanosomiasis (African Sleeping Sickness and South American Chagas Disease); these are caused by species of ''Trypanosoma''. Leishmaniasis is a trypanosomal disease caused by species of ''Leishmania''.
A variety of different forms appear in the life-cycles of trypanosomes, distinguished mainly by the position of the flagellum:

Amastigote (leishmanial)	- reduced or absent
Promastigote (leptomonad)	- anterior of nucleus, free from cell body
Epimastigote (crithidial)	- anterior of nucleus, connected by a short undulating membrane
Opisthomastigote (herpetomonad)	- posterior of nucleus, passing through a long groove in the cell
Trypomastigote (trypanosomal)	- posterior of nucleus, connected by a long undulating membrane

All trypanosomes have at least amastigote and promastigote stages. ''Trypanosoma'' appears in all five forms, with the trypanosomal stage occurring in the vertebrate host. ''Trypanosoma brucei'' sub-species have two forms in the bloodstream of a vertebrate host, the rapidly dividing long-slender form and the non-dividing short stumpy form. The short stumpy parasites are adapted for uptake into the tsetse fly vector, and are non-proliferative in comparison with the slender forms.
Unique to ''Trypansoma brucei'' is the expression of a variable surface glycoprotein (VSG) coat on the cell surface, which undergoes constant variation in order to evade the humoral immune system and host antibodies. It is thought that recombination from a repertoire of >1000 VSG genes is responsible for the vast diversity of the parasite, and its effectiveness in immune evasion.