They can blight the best holiday if, after a long day on the beach, you are looking forward to a balmy evening with a drink in your hand, watching the sun go down, and you are driven indoors by insects.
Mosquitoes are attracted to the sight, smell and heat of human beings. They seem to prefer some people over others: adults to children, big people to small, and men to women. They like the carbon dioxide we exhale and are attracted to the lactic acid produced in our muscles. People who sweat little are less attractive to mosquitoes than sweaty people.
There are also many other chemical compounds in our breath and on our skin that attract mosquitoes. Some perfumes, soaps and hair products make it more likely that mosquitoes will find you.
The first line of defence is to check what you are sitting under. Mosquitoes are attracted by carbon dioxide which is emitted in large quantities by patio heaters. Avoid them.
Citronella candles, containing a lemon-scented plant oil have, sadly, no effect, according to a study in The New England Journal of Medicine. Citronella lotion does work, however, when applied to the skin – but only for 20 minutes or so.
Much more effective is insect repellent containing at least 30 per cent Deet, which offers the best protection, according to the London School of Hygiene and Tropical Medicine. This is essential in tropical countries affected by malaria, where long trousers and long sleeves are de rigueur in the evenings and mosquito nets are required at night.
Malaria is a problem in the tropics and there are about 2,000 cases a year in Britons returning from abroad, including some deaths. Anti-malaria drugs should be taken before, during and after travelling to these countries.
Sunburn and heatstroke
It is the first thing the typical pale-skinned British holidaymaker does on arriving at their destination – race for the beach to broil in the sun. It is also the quickest route to skin cancer and to accelerated ageing.
Nothing ages the skin faster than exposure to the sun, which damages cells in the epidermal layer and reduces their elasticity, hastening the development of wrinkles. When a group of leading British dermatologists were asked at a press conference last month to name the most effective anti-ageing cream on the market, they responded unanimously: suntan lotion.
Slapping on the sun cream and sheltering when it is at its height between 11am and 3pm can also protect against melanoma, the severest form of skin cancer, which causes 1,800 deaths a year in the UK. The cancer has increased more than 40 per cent in the last decade and is rising fastest in young people in their teens and twenties. It is commonest in the better off, who are more likely to holiday in exotic locations. Severe sunburn before the age of 15, is a key trigger for the disease. Children (and adults) with fair skin are at the greatest risk.
Sunburn can also lead to heatstroke, when the body becomes rapidly over-heated. Heatstroke may also result from extreme physical exertion or very high temperatures and causes fever and reddened, dry skin. Extreme heatstroke, when the body temperature rises above 41C, can cause damage to the brain, liver and kidneys. Drink plenty of water and avoid physical exertion in extremely hot weather.
If you suffer from a tummy upset, it isn't necessarily caused by food poisoning. Your digestion can be disturbed when local bacteria in the food and water replace the bacteria that usually live in your bowels without causing any harm.
To minimise the risk, drink bottled water, avoid shellfish, avoid ice cream and ice-in drinks, wash salad vegetables in bottled water and, if you're eating hot food, make sure that it is piping hot.
These are also useful precautions against gastroenteritis, which may be caused by bacteria, viruses or parasites. The usual symptom is diarrhoea, which may be accompanied by nausea and vomiting, both caused by the excess of unabsorbed fluids in the intestine.
As a result of the loss of fluid through diarrhoea, the main complication of gastroenteritis is dehydration. Sufferers should drink at least two litres of water per day, plus a glass (200ml) each time they go to the loo (pass a loose stool).
Diarrhoea can be treated with loperamide, available without a prescription, also available under the brand name Imodium. It does relieve the effects but should not be taken by pregnant women.
Scientists are developing a vaccine patch to prevent travellers' diarrhoea, which would be worn on the skin for a few hours a couple of weeks before travel. Early trials have shown it sharply reduces the attacks, but it is not yet on the market.
Bites and stings
A nasty sting can ruin a day out. In the UK, though they may be painful, they are mostly harmless. However, 3 per cent of people are at risk of anaphylactic shock, a sudden reaction which can lead to swelling, loss of consciousness and death. This normally only follows wasp stings and susceptible individuals should carry a syringe of adrenaline to be injected as an antidote.
In other countries, there may be other threats from jellyfish, scorpions, snakes and spiders. The Mediterranean has been plagued by swarms of stinging jellyfish in recent years and a new warning was issued this week about shoals of "mauve stingers" spotted between Corsica and the French coast. From next week, Cannes will start erecting booms to try to protect its beaches. Mauve stingers can inflict a nasty sting which may leave a weal across the skin but are not usually serious. In the Mediterranean, it is advisable to wear goggles to check for jellyfish as you swim.
If you are stung, the traditional remedy is to pee on the area (or get someone else to do so) because the alkaline urine will neutralise the acid sting. Or use a solution of sodium bicarbonate (not often to hand on a beach). It is unlikely to help much, however, as the sting venom is already deep in the dermis and anything applied to the surface of the skin won't penetrate that far (it may have a minor role in neutralising any tentacles left sticking to the skin). The matter is complicated by the fact that some species of jellyfish have alkaline stings, and need acid such as vinegar to neutralise them. The safest option is to douse the affected area in sea water, offer a painkiller such as paracetamol and lots of TLC.
You have spent many hours on an aircraft and you do not have many days on the ground at your destination. The one thing you want to avoid is spending them feeling groggy and under par. But resetting your body clock after passing through several time zones is tricky. One report last year suggested that Viagra was the answer – but the drug had only been tested on hamsters, and in Argentina. Some travellers swear by Provigil, the drug for the sleep disorder narcolepsy, which has been tested by the military as a means of keeping troops alert in battle. Others use melatonin, the food supplement, widely believed to help jet lag, but lacking evidence that it does so.
Doctors urge caution over the use of powerful drugs which, while they may keep you awake, have not been shown to adjust the sleep cycle. The best natural defence against jet lag is to get as much sleep as possible before and during the flight and to make small adjustments to the new time zone. Once arrived, take a walk. The light, fresh air and exercise can help reset your body clock.
Taking naps helps to recharge the batteries, provided they do not last more than 45 minutes, to avoid the deep sleep from which it is harder to recover. The stimulant caffeine takes 15 to 30 minutes to work, so a cup of coffee before a nap should mean you wake with a spring in your step.
For many in the 18-30 generation, holidays and sex are inseparable. One study of holidaymakers in Tenerife found that half of those aged under 25, and almost a quarter over 25, had sex with someone new while on holiday.
Some gynaecologists argue that holiday sexual liaisons between new partners are now so common that travellers should be routinely tested for sexually transmitted diseases on their return.
Writing in the British Medical Journal in 2004, Karen Rogstad, a consultant in the genito-urinary medicine department at the Royal Hallamshire Hospital, Sheffield, said the alarming increase in STDs made any sexual encounter "potentially hazardous".
"Exposure to new sexual networks, the rate at which partners are changed while away, lack of condom use and consumption of alcohol increase the risk," she said.
One in five cases of syphilis among heterosexual men and more than two-thirds of cases of HIV in British-born men, and a quarter in women, were contracted through sex while abroad.
Tour operators should supply advice on safe sex and anyone who had sex with a new partner while on holiday should be checked for STDs once they returned to the UK.