Why do mosquitoes bite me and not the person next to me?

Have you ever been in bed with a friend, only to wake up covered in bites while they sleep peacefully? It feels like the mosquito has a GPS set to your name.

Here is the truth: mosquitoes do not choose you because of your blood type. They choose you because of three things — the carbon dioxide you breathe out, the smell of your skin, and your body heat.

First, long-range navigation. A mosquito's antennae carry special sensors that can smell carbon dioxide from tens of metres away. Every breath you take paints a trail in the air, and the mosquito rides that trail like a surfer on a wave.

Second, mid-range scanning. Once it is close, it switches to smell. Your skin releases hundreds of chemicals — lactic acid, uric acid, carboxylic acids. But here is the twist: these smells are not your sweat itself. They are what the bacteria living on your skin make after eating your sweat. Everyone's bacteria are different, so everyone smells different to a mosquito. To some people, your smell is a dinner bell. To others, it is a warning sign.

Third, heat check. Before it lands, the mosquito feels your body heat with the tip of its antennae. Warm skin is a green light.

So who gets bitten most? People who breathe heavily, people who have just exercised, and pregnant women. A pregnant woman breathes out about 20 per cent more carbon dioxide and runs a little warmer, which makes her roughly twice as attractive to mosquitoes as other adults.

What about blood type? Some studies found that people with type O blood get bitten slightly more often. But the effect is tiny compared with carbon dioxide and body smell. It is a real but overblown correlation.

Only two kinds of defence actually work: repellents containing DEET or picaridin, and physical barriers such as mosquito nets and long sleeves. Vitamin B1, mosquito-repelling bracelets, and ultrasonic devices? The US Environmental Protection Agency and several university studies have tested them and found they do nothing.

Try this tonight: wash your feet carefully with soap, scrub between the toes, then put on clean thin socks. The bacteria on your feet produce some of the strongest mosquito-attracting smells on your body. Washing them away works better than spraying repellent all over your skin.