WEATHER | DISEASE | WATER | HUNGER | AIR POLLUTION | ALLERGIES 
Infectious diseases caused by mosquitoes, ticks and fleas (vectors) are spreading because of global warming. Most vector-borne diseases—such as malaria, dengue fever, yellow fever, and encephalitis—exhibit a distinct seasonal pattern, with weather variables such as temperature and rainfall affecting both the vectors and the disease-causing pathogens they transmit.
Mosquitoes, for example, are very sensitive to temperature changes. Higher temperature increases their rate of reproduction, the number of blood meals they take, prolongs their breeding season, and shortens the maturation period for the pathogens they carry. Rising global temperatures also results in the expansion of mosquito and other vectors into areas with previously unexposed populations.
A U.S. study found evidence for a variety of routes for global warming to adversely affect disease spread. For instance, warmer winters could reduce seasonal die-off of many pathogens and their carriers, or allow them to move into areas that were previously too cold. Other possibilities include the spread of pathogens that thrive on warmer water, the joining of pathogen and potential hosts populations previously separated by climate factors.
Human diseases whose spread has been connected to warming, include malaria, Lyme disease, yellow fever and others. Most involve the expanded range of carriers like mosquitoes into higher latitudes. In the Hawaiian Islands, for example, in the 1960s mosquitoes were restricted by temperature to elevations below 2,500 feet, but warmer temperatures have allowed them to move higher up the mountainsides. With the mosquitoes has come the spread of avian malaria to native bird populations.
West Nile Virus' expansion to the United States can be explained by northern migration of insect borne tropical diseases as winters get hotter and there is less opportunity for insects to die off the way they normally do in nature. The 1999 outbreak of West Nile virus in New York in which seven people died, and the subsequent expansion of the disease in the summer of 2002, exemplifies what may occur more regularly as global climate change progresses.
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