These tiny little creatures breed in the dirty water ponds and pits which can be found every where around, thanks to our so called "civilized" citizens. From the eggs that are breed-ed these mosquitoes invade the world outside! 
However like someone said, something you may not know is they have the deadliest bite known to man. A shark might bite you in half, lions can tear you limb from limb and a snake can inject you with a toxin that will leave you a quivering mass in minutes. Nevertheless, they just don't have the numbers. More people die every year from the bite of the lowly mosquito than the bites of all other beasts put together.Why ?
These nasty little insects make buzzing sounds around the ear when i try to sleep after late night study/work
.Sometimes I waved my hands vigorously in vain and silly attempts to chase away the hungry things .. if i'am lucky Gotcha ! 
you are not going back in the air my little foe ! and if i'am not other foe bites off my other hand sucks mouthful of blood leaving an irritating itch.sure it is not most powerful bit that human's can have but its more unexpected in timing and number.
And Not to say while driving these things hit straight to face 
eww! I remember once we had an electric mosquito swatter bat which is used to kill those mosquito's at home.In one wave many mosquito's would be killed giving out "phat! phat!" 
sounds But one fine day these mosquito's even Broken that bat and that day i came to know that these mosquito is stronger that the China's Electric bat ! 
Some times if we even wave to scare that thing off,it again comes for more.i think either the mosquito is not so clever to understand that this time it will definitely gets a blow if it returns to the same target or the mosquito has more will power and guts to try again and again or even confused about target.I heard somewhere that Mosquito's have some special sensors to detect the scent of the humans and when mosquito gets closer it then uses infrared sensors to detect body heat, in that case they should at least sense the difference in body heat of different targets right ? may be my second reason is right and these things do have guts ! 
How ?
Instead of using Mosquito Repellents in every home and places you go,these mosquito's must die from the natural causes ! Most of the humans who travel in these pollution environment effected by different types of harmful smokes and gases from vehicles and factories gets effected daily then,why not mosquitoes.. they should die because of these harmful gases.Mosquitoes should die by the effects of the global warming.when so many beautiful birds like sparrow and all are getting instinct by the effects of cellphone waves and radiations why not mosquito's !
The eggs they lay in the water should be toxicated by the salts and chemicals in water and they should not breed.In a old telugu film i saw. One of the role Brahmanandham garu played says " 75 kilos maa amma ni oka chinna Eega champinappudu, oka chinna eega ni nenu champakoodada" (which Translates "when an small housefly can kill my mother who weighs 75 kilos the why cant i kill an small housefly" in reply to a priest saying "killing small creatures is a sin")
I think the same logic applies to the mosquito too
so the Mosquito must die ! 
