They're behind more human deaths than any other animal, but mosquitoes themselves are actually pretty harmless. Males eat a vegan, nectar-based diet, and although egg-laying females drink blood to get protein, even they don't cause much trouble besides red, itchy welts. The real risk from mosquitoes is the diseases they carry.
Female mosquitoes transmit a wide range of diseases among their hosts, ranging from malaria — a parasite that kills more than 3 million people a year — to dengue fever, yellow fever and West Nile virus. The dangers posed by these and other mosquito-borne diseases are expected to grow as temperatures and rainfall increase in much of the world, including parts of the U.S.
See the video below for an up-close look at how mosquitoes feed on human blood: