Parasitic protozoan. Coloured scanning electron micrograph (SEM) of trypanosome protozoa (Trypanosoma sp.) in a blood smear. Trypanosomes undergo part of their development in the blood of a vertebrate host. Two species, which are transmitted through the bites of tsetse flies, cause sleeping sickness (African trypanosomiasis). Another species, which is transmitted by reduviid insects, causes Chagas' disease. Both diseases may be fatal if not treated.