Tropical diseases are infectious diseases that are prevalent in or unique to tropical and subtropical regions. These diseases are less prevalent in temperate climates, due in part to the occurrence of a cold season, which controls the insect population by forcing hibernation during the cold season. Insects such as mosquitoes and flies are by far the most common disease carrier or "vector". These insects may carry a parasite, bacterium or virus that is infectious to humans and animals. Most often disease is transmitted by an insect "bite", which causes transmission of the infectious agent through subcutaneous blood exchange. Vaccines are not available for any of the diseases listed here. In 1975 the United Nations Children's Fund, the United Nations Development Programme, the World Bank and the World Health Organization established the Special Programme for Research and Training in Tropical Diseases (TDR) to focus on neglected infectious diseases which disproportionately affect poor and marginalized populations in developing regions of Africa, Asia, Central America and South America. The current TDR disease portfolio includes the following ten entries:                Malaria                African trypanosomiasis                Dengue fever                Leishmaniasis                Schistosomiasis                Tuberculosis                Chagas disease                Leprosy                Lymphatic filariasis                Onchocerciasis                Trachoma