Tsetse are biological vectors of trypanosomes, meaning that in the process of feeding, they acquire and then transmit small, single-celled trypanosomes from infected vertebrate hosts to uninfected animals. Some tsetse-transmitted trypanosome species cause trypanosomiasis, an infectious disease. In humans, tsetse transmitted trypanosomiasis is called sleeping sickness. In animals, tsetse-vectored trypanosomiases include "nagana", "souma" (a French term which may not be a distinct condition), and "surra" according to the animal infected and the trypanosome species involved. The usage is not strict and while "nagana" generally refers to the disease in cattle and horses it is commonly used for any of the animal trypanosomiases.