In 1894, Colonel David Bruce discovered that the fly was not in itself poisonous but that the deadly effect of its bite was due to the fact that it transmitted a highly pathogenic blood parasite, _Trypanosoma brucei_. This trypanosome Bruce had discovered in the blood of South African cattle suffering from a highly fatal disease known as "nagana". On inoculating the blood of infected cattle into horses and dogs he produced the disease and found the blood teeming with the causative organism. In the course of his work he established beyond question that the "nagana" and the tsetse-fly disease were identical. Tsetse-flies of the species _Glossina morsitans_, which fed upon diseased animals, were found capable of giving rise to the disease in healthy animals up to forty-eight hours after feeding. Wild tsetse-flies taken from an infected region to a region where they did not normally occur were able to transmit the disease to healthy animals. It was found that many of the wild animals in the tsetse-fly regions harbored _Trypanosoma brucei_ in their blood, though they showed no evidence of disease. As in the case of natives of malarial districts, these animals acted as reservoirs of the parasite. Non-immune animals subjected to the attacks of the insect carrier, quickly succumbed to the disease. A question of prime importance is as to whether the insect serves as an essential host of the pathogenic protozoan or whether it is a mere mechanical carrier. Bruce inclined to the latter view. He was unable to find living trypanosomes in the intestines or excrements of the fly or to produce the disease on the many occasions when he injected the excrement into healthy animals. Moreover, he had found that the experimental flies were infective only during the first forty-eight hours and that if wild flies were taken from the infected region, "kept without food for three days and then fed on a healthy dog, they never gave rise to the disease."