Carbon dioxide is one of the biggest clues. Every breath releases CO₂, and mosquitoes can use it to detect that a potential host is nearby. Body size, metabolism, and physical activity can influence how much you produce. Exercise can make you particularly noticeable because you’re simultaneously breathing faster, producing more carbon dioxide, generating additional body heat, and sweating. Mosquitoes can also detect warmth and moisture, giving them several signals to follow at once.

Your skin chemistry matters too. Sweat, natural skin oils, and microorganisms living on your skin create a complex chemical signature that differs between individuals. Certain combinations appear more attractive to some mosquito species than others. Genetics may influence these characteristics as well. Blood type has also been studied, with some research suggesting differences in attraction, but it isn’t a simple rule and certainly doesn’t determine whether you’ll be bitten.

Clothing and circumstances can add to the effect. Dark clothing may provide stronger visual contrast, while warm, humid conditions encourage mosquito activity. Pregnancy can alter body temperature, metabolism, and carbon-dioxide output, potentially increasing attraction under some circumstances. Alcohol has also been investigated, although evidence remains limited. Another complication is that people react differently to bites: one person may develop large, obvious welts while another experiences barely visible marks despite receiving bites.

Fortunately, you don’t need to change your biology. Proven insect repellents used according to their directions, protective clothing, window screens, mosquito nets where appropriate, and removing standing water around the home can reduce bites. Avoid excessive scratching, since broken skin can become infected, and seek medical attention for serious allergic reactions or unusual symptoms.

So if mosquitoes always seem to find you first, they aren’t holding a grudge—your particular combination of heat, breath, scent, and skin chemistry may simply make you easier to detect.