Mosquitoes are primarily attracted to carbon dioxide, a waste product exhaled from the lungs. They also seek out warm, moving bodies. If given a choice, they prefer to bite people who produce high levels of lactic acid, uric acid or cholesterol byproducts in the skin.
Larger people and pregnant women emit more carbon dioxide and attract more mosquitoes. People who are exercising are highly attractive to mosquitoes, as warm, moving bodies emit large amounts of carbon dioxide and lactic acid. Dark clothing allows mosquitoes to spot victims more easily.