Insect bites are the most common complaint among returning British holidaymakers, according to a report commissioned by the Royal Pharmaceutical Society — which neatly sums up the gap in health priorities between developed countries and the rest of the world. Affluent tourists worry about being bitten alive as they sip cocktails by the pool, while most of the rest of the world fears being bitten to death thanks to insect-borne diseases such as malaria, yellow fever and encephalitis.

The toll exacted by such diseases — malaria alone kills a million people a year — has made this area a priority for researchers and we now have a much better understanding of how certain insects select their prey.

US scientists recently identified three sets of receptors that mosquitoes use to find their next meal. One set detects carbon dioxide exhaled in our breath, another picks up traces of octenol (a chemical found in breath and sweat) and the third “sniffs” a brew of some of the other 350 compounds that make up body odour.

Mosquitoes pick up carbon dioxide downwind from their victim, then make their approach upwind along a rising concentration gradient — the higher the CO2 level, the closer they are to their target.

We all produce similar levels of carbon dioxide but the mix of other chemicals in our body odour is as individual as fingerprints, and it may be this that determines whether you or your partner is bitten. Put simply, if two people are sitting side by side, the carbon dioxide that they exhale draws in the mosquito but subtle nuances in individual body odour can influence its final approach.

The other reason why some people appear to be bitten more than others is that there is tremendous variation in the immune response to a bite. The classic itchy red lump is caused by our immune system mounting a defence against the anticoagulants that insects inject to ensure that they get a decent meal before our blood starts to clot. If you have never been bitten by a particular species of insect before, your reaction to the first few bites will be fairly minimal — indeed, you may not even notice them. And if you live in a part of the world where being bitten is an everyday occurrence, your immune system eventually “gets used” to the foreign chemicals and the reaction starts to subside. It is those in the in-between group — ie, most of us — who are likely to react most, and the more you are bitten the worse the reaction is likely to be (until it peaks and eventually starts to wane).

So what is the best way to keep insects at bay? First, don’t make yourself too attractive. Research suggests that people wearing perfumes and aftershave are more likely to be bitten, as are beer drinkers.

Second, use a good repellent such as N,N-diethylmetatoluamide (DEET), which blocks the insects’ receptors and makes wearers “invisible”. Citronella is a popular natural alternative but it is nowhere near as effective. One study comparing 16 commercially available repellents found that products containing 24 per cent DEET protected volunteers for an average of just over five hours when they put their arms in a tank full of mozzies, while those using 10 per cent citronella (the strongest tested) received their first bite within 20 minutes. Citronella-impregnated wristbands offered protection for just 20 seconds.

Laboratory tests have shown that, contrary to travellers’ lore, neither garlic nor vitamin B (thiamine) supplements offer any useful protection.

And don’t forget to cover up where possible. Ankles and wrists are favoured targets, particularly during the evening, and the best way to protect yourself in bed is to sleep under a mosquito net.

If, despite all the above, you are still bitten — and you probably will be — try not to scratch, as it worsens the irritation and can lead to infection. The best treatment for bites is a combination of 1 per cent hydrocortisone cream and a long-acting non-sedative antihistamine (both available over the counter from a pharmacy). Take the antihistamine and apply the cream as soon as you are bitten, reapplying every 12 hours.

Or you could rub the bite with an onion. So says a press release from the British Onion Producers’ Association that landed on my desk this morning. Evidently the “world-renowned onion chemist” Dr Eric Block has found that natural compounds in onions break down the chemicals responsible for inflammation and discomfort. Indeed, Dr Block suggests that “everyone takes an onion on their picnics this summer” — advice that I feel duty bound to pass on. Just make sure it is a British one.