Tropical infections thrive in the tropics' hot and humid climate. Viruses, bacteria, and parasites cause them, and they spread by airborne transmission, sexual contact, and contaminated food and water sources. Through bug bites, insects or other animals can spread tropical infections. Neglected tropical infections afflict the world's poorest countries, affecting around 1.4 billion people in 149 countries. Every year, they cost poor countries billions of dollars. In temperate regions, diseases are less common, owing in part to the presence of a cold season, which limits insect populations by forcing hibernation. Exploration of tropical rainforests by humans, deforestation, rising immigration, and greater international air travel and other tourism to tropical regions have resulted in an increase in the occurrence of such diseases in non-tropical countries.