Until nearly the end of the nineteenth century about two-thirds of Southern Rhodesia was uninhabitable by domestic cattle, and thus, to a great extent, unusable by mankind whether European or native, because of the disease trypanosomiasis. In man this is called sleeping sickness, in cattle nagana. It is caused by trypanosomes, minute organisms which live in the blood of the victims of the disease. These trypanosomes are sucked up, with the blood of the infected man or beast, by blood-sucking tsetse flies, develop in the fly and may be transmitted to the next creature on which the fly feeds.