Tropical diseases are diseases that are prevalent in or unique to tropical
and subtropical regions. The 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. However, many were present in
northern Europe and northern America in the 17th and 18th centuries before
modern understanding of disease causation. The initial impetus for tropical
medicine was to protect the health of colonial settlers, notably in India
under the British Raj. 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.