Some people get ravaged by mosquitoes if they so much as take a walk at dusk. Others can walk through clouds of the insects and not get a single bite. What's the difference? A lot. Scientists have figured out many reasons why mosquitoes can't seem to resist some people, but are repulsed by others.
Mosquitoes are sources of severe infection in many parts of the world, there has been a lot of research done about why some people are mosquitoes' favorite snack.
It starts with the types of people who attract mosquitoes in the first place. Although mosquitoes can smell blood for miles, they mostly locate their prey by tracking the carbon dioxide that animals exhale. People who exhale more carbon dioxide - large people and pregnant women - are more obvious targets. There is also evidence that mosquitoes prefer women because their skin is thinner, allowing for an easier bite.
Mosquitoes also uphold the old joke about vampires looking for people who are their "type." Different blood types either attract or discourage mosquitoes. If you have found yourself the sole person among a camping group bitten up, you're probably an O blood type. A group of scientists in Japan exposed people to groups of mosquitoes which had had their biting and sucking parts removed. The mosquitoes landed again and again on the O blood types, ignoring the A and B blood types.
Mosquitoes also have a sweet proboscis. They prefer people whose body chemistry secretes saccharides, which people who remember saccharin know taste sweet, on their skin. Then again, the saccharides might only be involved because they feed other animals that encourage mosquitoes. Bacteria live on the skin, and help give sweat its disgusting smell. Dutch scientists found that a high abundance of only a few types of bacteria encourage mosquitoes. A large variety of bacteria living on the skin, though, tend to discourage mosquitoes, as does a low overall number of bacteria. Basically, you want your skin to be an island ecosystem. There aren't a lot of overall animals, but there is a diverse range of unique species.
Some scientists and companies are attempting to analyze and combine the scents of these elements to manufacture a bug spray that will effectively deter mosquitoes. (I wouldn't mind wiping myself down with harmless skin bacteria if it would keep mosquitoes off me.) However, going is slow, and mosquitoes are canny. Hopefully, those with O blood types who have noticed that their skin tastes relatively sweet will simply decide never to venture into nature again.
Via Eyes on Malaria, The Straight Dope, PLOS, and Library of Congress.