Mosquitoes are the deadliest insects on the planet, not because of the mosquito itself, but because of the diseases that they carry and transfer to their hosts while feeding on the host's blood. Mosquito-born disease is responsible for the death of millions of people each year; an estimated 70 million are infected through mosquito bites annually. Diseases they carry include malaria, yellow fever, dengue, encephalitis, and West Nile virus.
The most common species of mosquitoes typically are no larger than .5", but size varies greatly depending upon the specific type of mosquito. They are nocturnal, and will usually spend the hottest part of the day in cool areas; they typically will not bite unless disturbed during the day. They can fly for up to 4 hours at a time, and may cover a range of 6.25 miles in a single night.
A mosquito bite is, of itself, not dangerous. It will swell, redden and itch for a few hours to several days, but will go down on its own. If you begin to experience any symptoms of illness after being bitten, seek medical attention immediately.
Mosquito control requires a combination of several steps, including reducing or eliminating breeding areas (such as stagnant water), the application of larvicides to and around breeding areas, and the application of insecticides to areas in which mosquitoes are present during the heat of the day (shrubs, overgrown areas, etc.).