By trypanosomiasis is meant a condition of animal parasitism, common to man and the lower animals, in which trypanosomes, peculiar flagellate protozoa, infest the blood. Depending upon the species, they may be harmless, producing no appreciable ill-effect, or pathogenic, giving rise to conditions of disease. A number of these are known to be transferred by insects.

The trypanosomes are elongated, usually pointed, flagellated protozoa in which the single flagellum, bent under the body, forms the outer limit of a delicate undulating membrane. It arises near one end of the organism from a minute centrosome-like body which is known as the blepheroplast, and at the opposite end extends for a greater or less distance as a free flagellum. Enclosing, or close beside the blepheroplast is the small kinetonucleus. The principal nucleus, round or oval in form, is situated near the center of the body. Asexual reproductions occurs in this stage, by longitudinal fission, the nucleus and the blepheroplast dividing independently of one another. From the blepheroplast of one of the daughter cells a new flagellum is formed.