Selection
Mosquitoes are attracted to sweat and particularly sweat that produces more carbon dioxide and other compounds that make up body odor than others. This explains why mosquitoes seem to be more attracted to some people over others. Mosquitoes have a very finely tuned sense of smell that is used to find their victims. They have 72 olfactory (smelling) systems in their antennae and, of these, 27 are specifically designed for sniffing out their victims. They will fly around until they pick up a scent, and then zero in on their target.
Time and Place
Mosquitoes like hot, moist climates. Most mosquitoes are crepuscular (dawn or dusk) feeders and will find a cool place to shelter during the heat of the day. Most prefer to bite animals and birds rather than humans. They are also known to be excellent at sniffing out their victims and going to great lengths to obtain access to them, such as infiltrating buildings. They can find their way through very small openings; if there is a way into a place to get to their victim, they will find it.
Saliva
To combat the vertebrate's defenses when it comes to blood, the mosquito injects its saliva into the area about to be bitten. When blood is shed on mammals, it is designed to coagulate and congeal to form into clots so as to stop it flowing. This defense mechanism would clog the mosquito's proboscis, so it has anti-clotting chemicals in its saliva to prevent clotting allowing it to feed efficiently.
Disease
Mosquito bites are notorious spreaders of diseases, with the most common the deadly malaria. Mosquitoes are vector agents: they carry disease-causing viruses and parasites from human to human without getting the diseases themselves. They also commonly carry dengue fever and yellow fever. They carry diseases to an estimated 700 million people a year. They can be limited in their effects by mosquito-eradication programs, vaccinations and prophylactic drugs.