(a) : Syphilis is caused by a spirochete (spiral bacterium) Treponema pallidum. Sleeping sickness is a disease of tropical Africa caused by the presence in the blood of the parasitic protozoan, Trypanosoma gambiense. Plague is an epidemic disease of rats which is transmitted to humans by rat fleas. Dengue is a disease transmitted to humans by mosquito, Aedes aegypti.