Malta fever was shown by Bruce to be due to Micrococcus (Brucella) melitensis. The disease was later named Brucellosis.

In 1895 Bruce found that nagana, the tsetse fly disease of Zululand, was due to a trypanosome (T. brucei). He described a hematozoon in the blood of the affected animals that had not been previously described.

Digital facsimile from the Internet Archive at this link.

In this more-detailed follow-up to his "preliminary" paper of 1895 published in Durban, South Africa, Bruce provided definitive proof that the Trypanosoma was the cause of nagana, and the tsetse fly was the vector of transmission. He showed the part of the tsetse fly which took part in causation of nagana, described the trypanosoma, proved the connection between big game and spread of the disease, and provided a treatment for the disease (prophylactic and curative) using arsenic in the animals. 

In this paper Bruce also credited Lady Bruce, a trained laboratory technician, for help with the experiments. Sir David and Lady Bruce comprised the complete team in the "First British Nagana Commission to Zululand." Digital facsimile from the Internet Archive at this link.

(Thanks to Juan Weiss for this reference and its interpretation.)

Bruce and D.N. Nabarro were sent to Africa by the Royal Society to study sleeping sickness, and in their report they showed that the tsetse fly was the vector of trypanosomiasis. They also found that Gambia fever and sleeping sickness were two stages of the same infection.