David Bruce. Portrait of Sir David Bruce (1855- 1931), Scottish microbiologist who specialised in tropical diseases. Bruce joined the Army Medical Service in 1883. In 1887 he isolated the bacterium which caused undulant fever or brucellosis. He later showed that it was spread by unpasteurised milk. In 1894 he studied nagana, a cattle disease. He showed that it was caused by a protozoan parasite (Trypanosoma brucei) which was spread by the bite of the tsetse fly. He later showed that T. brucei was also transmitted by the tsetse fly to cause sleeping sickness (trypanosomiasis) in humans. He was made Chairman of the War Office's Pathological Committee during the First World War.