Why mosquitoes bite some people and not others

Stage one: the plume

A mosquito can detect carbon dioxide in a plume of air and follow it upwind. Your breath is a beacon, and so is the carbon dioxide that leaves your skin.

At this range it is not choosing you personally. It is simply flying toward a concentration gradient.

Stage two: the shape

Within about ten metres, vision takes over. A mosquito heads for a dark, moving shape against a lighter background.

This is why dark clothing attracts more bites than light clothing, and why a person standing still in a pale shirt is less interesting than someone moving in navy.

Stage three: the landing

At close range the mosquito reads body heat and humidity, then the chemistry of your skin: lactic acid, ammonia, and the volatile compounds that skin bacteria produce from sweat.

That last part is why two people in the same garden have different experiences. The bacterial community on your skin is as individual as a fingerprint, and it changes what you smell like to an insect.

Other factors show up in studies again and again: larger bodies produce more carbon dioxide, pregnancy raises both breath volume and skin temperature, and alcohol changes what the skin gives off.

Why some bites swell and others do not

The itch is an immune reaction to proteins in the mosquito saliva, which contains an anticoagulant.

People who have lived with a species for years often stop reacting, which is why visitors to a new region can be covered in bumps while residents are barely marked.

Where they come from

Adults live for a few weeks. The larvae live in water, and they do not need much: a saucer under a plant pot, a blocked gutter, a tarpaulin fold, a bucket left out.

In warm weather the cycle from egg to adult takes about a week, which is why one neglected container can produce a steady supply all summer.

One clarification worth making: mosquito bites are not caused by dirty blood or by a sweet tooth. They are a protein meal taken by a female that needs it to lay eggs.