Tropical iseases 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.