About 10-20% of people get bitten by mosquitoes more than others, and it’s probably not because they’re "so sweet," like their grandmothers keep telling them. Genetics are the main reason mosquitoes prefer biting some people over others, but there are other circumstantial factors that come into play.
There are about 3,000 species of mosquitoes (150 different species in North America alone), and mosquitoes have been around for 170 million years, plenty of time to become experts in seeking out the best blood. Mosquitoes do not drink blood for sustenance, rather to help females develop eggs. They drink nectar to survive, and males do not drink blood at all.
85% of mosquito preference is related to a human’s genetics. They prefer people with the blood type O twice as much type B, and they like blood type A even less. They are also attracted to people with high concentrations of cholesterol or steroids.
Heat, movement, lactic acid from human sweat and carbon dioxide also alert mosquitoes to a human’s presence. They can even sense carbon dioxide from 100 feet away, which is bad news for anyone who is breathing harder than those around them. Pregnant women and people who are exercising or drinking beer are therefore more susceptible to mosquito attacks.